Signed-off-by: yuexin <yuecn41@gmail.com>

- remove Markdown_ prefix from s: functions
- always add one empty line between commands and methods

Fix #99.

Reverts #141 which @cirosantilli wrongfully merged.

Add regression tests that would have prevented that.

Correction pointed out by @shanesmith at #147.

:Toc causes errors because getqflist() returns empty for location
list.
Fix #108

Fix #100.

Fixes issue #102

If mappings aren't defined as 'buffer local', they are defined in other
files too and this causes conflicts with mappings there.

It breaks too often when you have code blocks with comments that
start with `#`, and no one uses spaces before headers.

The following error occurs with Vim 7.4.260.
E128: Function name must start with a capital or "s:"

Rename function names to `s:Markdown_`.

Fixes #79

Every non local identifier must start with g:vim_markdown_ from
CONTRIBUTING.md.