Mappings to add new items

• JSX support
• Rust support
• Support for items that span multiple lines
• Mappings for adding new items
• Lots of bugfixes

Multiline

The big feature is multiline support! Took a while to get right, but it finally seems like it works fine for all use cases. Experiment with, for example:

<div class="example"
     style="color: red;"
     something="other">
  Example
</div>

Apart from that, there's also support for handlebars components now, and a bunch of fixes and refactorings.