formatting: make custom indentexpr optional.
toc: handle some cases that caused errors

check if there is a valid locationlist, if there is a filename, prevent
some error messages
created `chdir` module.

Automatically change the window current directory to the one the current
file is in.
python/command: fallback to sync method if v:servername is null

sometimes, console vim doesn't register a servername, which caused the
Pandoc command to send invalid arguments to async.py.
command: implement command templates system.

also, reorganized menu
Merge branch 'master' of github.com:vim-pandoc/vim-pandoc