Toggle markdown flag in Simplenote to match current buffer filetype

Changes `update_note_from_current_buffer()` so that it sets the markdown
flag in Simplenote to match the filetype of the buffer. This means by
setting the filetype locally you can toggle the markdown status of a
note within Simplenote.

Also changes `create_new_note_from_buffer()` to set the markdown flag if
the buffers filetype has been set as markdown.

Needs further testing.

Need to squash this with previous commit 84c4ed8ee503f7456f153048c01b25f885bd3827
before merging upstream.

References this issue:

https://github.com/mrtazz/simplenote.vim/issues/49