Extend custom mode API with start/end and keyword mapping

[pepijnve]

This PR adds two extra capabilities to the custom mode rule Java API:

• start/end to support things like block comments
• keyword mapper support

With these two changes modes like the SQL mode can be completely defined via the Java API.

[pepijnve]

@F0rce I realised too late that the JS changes should have been done in https://github.com/f0rce/lit-ace instead of in this project. If you're OK with the proposed changes I'll split up this PR into one for lit-ace and one for this project.

[F0rce]

@pepijnve jeah, I should have mentioned anywhere, that JS changes have to be made in https://github.com/F0rce/lit-ace

The feature sound really interesting and I'll be sure to check it out as soon as the development is done. If you need help with the JS part, just ping me in the PR and I'll be sure to help you out. LGTM

[pepijnve]

F0rce/lit-ace#11 here you go. I can update this PR once there's a build containing that enhancement.