Add an Edit link to every generated page

[Hixie]

It would be great if every generated page of dartdocs could link to the relevant line on GitHub so that someone could really easily edit the docs and submit a PR for proposed changes.

P2 (Enhancements considered important but without significant data indicating they are a big win)

(From flutter/flutter#10416)

[jcollins-g]

This is largely a duplicate of #1454. I now have an implementation on deck good enough for Flutter's use (at least for Flutter-owned packages). The implementation will be able to link to an arbitrary URL constructed via a template and controlled via flags or dartdoc_options.yaml. From dartdoc's --help:

    --link-to-source-excludes                     A list of directories to exclude from linking to a source code repository.
    --link-to-source-revision                     Revision number to insert into the URI.
    --link-to-source-root                         Path to a local directory that is the root of the repository we link to.  All source code files under this directory will be linked.
    --link-to-source-uri-template                 Substitute into this template to generate a uri for an element's source code.
                                                               Dartdoc dynamically substitutes the following fields into the template:
                                                                 %f%:  Relative path of file to the repository root
                                                                 %r%:  Revision number
                                                                 %l%:  Line number

...and will have that link visible on all element pages next to the title.

The text <source code> will be replaced with an icon; checking with @kwalrath on what would be good for us to use there. Because the link is arbitrary, it could go to github's edit link as @Hixie suggest's, or simply view the source as requested in #1454.