Fixes bug where typing "*notalist* " will create a list

Fixes https://github.com/guardian/scribe-plugin-smart-lists/issues/14

Adds a check to make sure the cursor is in the right position when typing the space key to create a list.
Just a word of warning: in the past I've found range co-ordinates unreliable across both Firefox and Chrome – sometimes text nodes are split randomly, it seems, and these offsets are rendered useless.