Make the default mappings 'buffer local'

If mappings aren't defined as 'buffer local', they are defined in other
files too and this causes conflicts with mappings there.

Specifically, I discovered this issue after installing [vim-gitgutter](https://github.com/airblade/vim-gitgutter) which also uses `]c` mapping.
I tried the `]c` in a **ruby** file and it wouldn't work. I discovered this is due to `vim-markdown` having it's mappings defined **globally**.

Just to mention - I've read the contribution guidelines and I think these changes do not conflict with anything mentioned there.