Add support for disabling meta mappings

This adds support for g:rsi_no_meta. I wasn't sure whether:
- to check for g:rsi_no_meta before or after `set encoding=utf-8` (I decided to check before this is set)
- to document this setting in doc/rsi.txt (decided to document it)

Let me know if the PR can be improved.

Refs #14, #5