Fix an exception thrown when clicking the heading toolbar buttons twice

I believe this one is just a typo
Merge pull request #650 from rebelzach/fix-editor-buttons

Fix an exception thrown when clicking the heading toolbar buttons twice.
Fix broken issues link
Merge pull request #666 from xrd/patch-1

Fix broken issues link
Update README.md

A typo on the fourth word is hard to miss.
Plus some sentence refactoring (because I was already there)
Merge pull request #668 from michaeljoseph/patch-1

Update README.md
Add additional title attributes to links for enhanced screen reader accessibility.
Merge pull request #677 from ndarilek/master

Add additional title attributes to links for enhanced screen reader acce...
Request the v3 media type via the Accept header.
s/master_branch/default_branch
Basic stash retrieval for previews
Check for stashed content in preview
Reset to edit mode when using live preview
Whitespace cleanup
Change the way new preview's are handled
Revert "Change the way new preview's are handled"

This reverts commit 8d73488a9ea5d48b5c1465a6ad6fb1aaa75ecdaa.

I was mistaken about how the model was stored, due to a filename conflict while testing
More robust preview's for new files

Basically, when live previewing a new file:
- Store stash at generic file path of /parent-directory/new-file
- Preview that file, but slide a dummy model in if none exists, the preview function will pull from the stash first.

This is maybe a little kludgy and only works because, currently, the preview window is meant to be viewed and then closed to return to editing.
Remove extraneous console log
Stashing using absolute file path as key

Addresses prose/prose#519 Also cleaned up preview to use current filename
Update view path when saving