Include`custom.js` file if it exists

[budziq]

Hi similarly to https://github.com/azerupi/mdBook/issues/178

I would be interested in adding support for "custom.js" to be loaded if one exists. This two would let us stop needing to pull index.hbs (some minimal js there) and full book.css into rust-cookbook.

[azerupi]

Good idea, it should not be too difficult to add this to #287

[budziq]

Thanks :) . Issues such as rust-lang/book#55 could be implemented trivially downstream without any modifications to MDBook. If you don't feel like implementing it I'll gladly add it once https://github.com/azerupi/mdBook/pull/287 gets merged.

[earlAchromatic]

This doesn't work for remote urls. Say I want to import some external 3rd party lib? I should be able to do:

[output.html]
additional-js = ["./theme/custom.js", 'https://somedomain.com/dist/external.v1.js']

and have it add:

<script src="https://somedomain.com/dist/external.v1.js">

Even better if I can add notation for async and defer attributes.

Unless there is an existing mdBook way to do this?

[ehuss]

You can include any script tags you want in head.hbs, see https://rust-lang.github.io/mdBook/format/theme/index.html.

[earlAchromatic]

Got it thanks!