Add page layouts functionality

[jamos-tay]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] New feature

Fixes #389

What is the rationale for this request?

Users may want to add the same footer/head to a group of files at once without having to specify it in each file's frontMatter individually.

What changes did you make? (Give an overview)

Follow up from this solution: #389 (comment)

This PR adds support for MarkBind themes (changed from 'templates' because that name was used by nunjucks).

A theme consists of 5 files, navigation.md, head.md, footer.md, styles.css, scripts.js. Themes are stored in the _markbind/themes folder, laid out like this:

// This one is created on markbind init with all blank files - basically no change
_markbind/themes/default/
navigation.md, head.md, footer.md, styles.css, scripts.js

_markbind/themes/themeA/
navigation.md, head.md, footer.md, styles.css, scripts.js

_markbind/themes/themeB/
navigation.md, head.md, footer.md, styles.css, scripts.js

Users can apply themes to a set of pages either through the site.json or the frontMatter:

// Theme A will be applied to all index.md files
{
  "glob": "**/index.md",
  "theme": "ThemeA"
},

// Theme B will be applied to index2.md
{
  "src": "index2.md",
  "theme": "ThemeB"
},

// No Theme - default theme will be applied
{
  "src": "index3.md"
},

<frontmatter>
  theme: themeA <!-- Takes precedence over site.json -->
  head: head.md <!-- Takes precedence over themeA's head.md -->
</frontmatter>

All files in the folder will be applied.

Is there anything you'd like reviewers to focus on?

The following use cases aren't supported:

• Using default theme for certain files only: Overriding default theme overwrites all 5 files, so you can't have say default/head.md and myTheme/footer.md at the same time.
• Additive applying of scripts and css: Should a theme's js and css overwrite the default, or should it apply both?

[acjh]

Themes are better reserved for styling, e.g. #386.

How about "layout"?

[jamos-tay]

Sure, layout is fine

[yamgent]

Also right now the tests only includes overriding the layout in site.json. We can write additional tests to the test site for overriding the individual component of the layout in the <frontmatter> as well, e.g.:

<frontmatter>
  layout: layoutA
  head: head.md
</frontmatter>

[yamgent]

• Additive applying of scripts and css: Should a theme's js and css overwrite the default, or should it apply both?

This might result in several levels of css and js. It can go quite deep, making it hard for the user to control what should be included and what shouldn't be included.

The design can get quite complicated, but I think it is a feature certainty worth exploring in the future.

[jamos-tay]

Updated with fixes (+ additional test)

Sure, we can worry about that later

[jamos-tay]

Updated to resolve merge conflict

[yamgent]

I think it would be better to put these inside the markbind folder (no underscroll at the front) rather than inside the _markbind folder, because the _markbind folder has a connotation that it contains working files, rather than files for actual deployment.

Same thing for the JavaScript files.

[jamos-tay]

Sure, moved it to the assets folder.

Also added path to ignore so they don't get copied to the _markbind folder

[yamgent]

Seems to be repeating the same thing as The default layout is automatically applied to every single page. below?

Or do you mean it is generated when doing a markbind init?

[jamos-tay]

I meant to advertise the use case, like the purpose of templates is to quickly apply stuff to a batch of files.

I moved the sentence up so it's a little clearer.

[jamos-tay]

Necessary changes made!