doctest: specify ‘opening-delimiter’ so doctests are detected and run

[davidchambers]

Merging #9 inadvertently disabled the doctests.

This pull request is one of two possible fixes. The other option is to replace occurrences of ```js in index.js with ```javascript, allowing us to use the default opening-delimiter value.

If you would prefer to stick with ```js, Aldwin, you're welcome to merge this pull request. If you would prefer to avoid another .config entry, close this pull request and I will open a new one.

[Avaq]

Merging #9 inadvertently disabled the doctests.

Did the introduction of the opening-delimiter option change the "default" opening delimiter? That was not immediately obvious from reading the sanctuary-scripts release notes.

[Avaq]

Thank you! :)

[davidchambers]

Did the introduction of the opening-delimiter option change the "default" opening delimiter?

Prior to [email protected], doctest attempted to process every prefixed comment line. This proved problematic in sanctuary-js/sanctuary#600, because the Markdown syntax for block quotations is the same as the doctest syntax for input lines. To resolve this conflict we made it possible to specify, via delimiters, the regions in which doctest should attempt to process prefixed comment lines.