Exit blockquotes on return, stay in blockquote on shift+return

A proposal for dealing with blockquotes. On return, the caret is returned to the default block (paragraph, or `<br>`) and on shift+return, a `<br>` is inserted inside of the blockquote. 

Fixes #274, touches on #278.

Note: It appears that there were changes in master (src/) that weren't in the dist/ folder—you'll see a bit of noise from that. You can safely ignore the dist/ folder diffs.

Inspired by @percolate / @gmathieu / @mattkenefick's changes mentioned on #274.