Added option to disable all mappings. Fixes #34

Since I only needed `imap <Bar` for Cucumber, I've added this option and ended up with such .vimrc:

```viml
let g:table_mode_disable_mappings = 1
imap <Bar> <Plug>(table-mode-tableize)
```
When `g:table_mode_disable_mappings` is set to `1`, it means that mappings should not be added.