Allow a user to insert arbitrary HTML to header or footer of generated docs

[sethladd]

In order to allow a user to insert their own CSS or analytics snippet, we can allow them to add arbitrary HTML into the header or footer of each generated HTML page.

[keertip]

@sethladd, would it make sense to have a config file local to the user to specify these values and dartdoc will pick it up if available while generating the docs.

[sethladd]

Required are the necessary command-line args. A "nice to have" is a config file. But if we turn on command-line args, then people can script it however they want.

[keertip]

#328