capture enter key, create new sections

[bantic]

ready to merge

Adds Cursor model for semantically handling selection.
Adds reparseSection to the Post parser that can reparse markers.
Handles newline semantically -- capture the ENTER keydown event and split the section at the cursor and insert a new section.

To do:

• add tests for different actions in the editor (hit enter inside marker, hit enter at beginning/end of section, when in a section that is not the last section, etc)
• move cursor to new section after hitting enter
• handle deleting an entire marker (make text bold, delete to beginning of text and once more)
• handle deleting from beginning of a section into previous section
• bug: on FF, entering text in new section removes it

In future PR:

• remove extraneous Cursor methods
• handle hitting enter when text is selected
• Capture delete to control joining sections and preventing browser from generating HTML inside empty P tags
• Safari/FF bug: deleting an empty section moves cursor to weird place: