add option to enable insert mode leader mappings with a default off off ...

This is a follow-up to #8 which defaulted to having `g:markdown_enable_insert_mode_mappings` disabled.

After further investigation, I've found that there is a more friendly avenue as follows:

- Add a new option `g:markdown_enable_insert_mode_leader_mappings` which has a default of `0`.
- Default `g:markdown_enable_insert_mode_mappings` to `1` like it was before.

This means that OOTB, a user gets the nice default handling of:

1. [un]indenting empty list items on [s-]tab.
2. removing empty list items on <CR>.
3. formatting tables.
4. normal and visual mode leader mappings for block editing (<leader>e) and table formatting (<leader>ft).

This also means that if a user wants to have the insert mode leader mappings, they only need to do:

```vimscript
let g:markdown_enable_insert_mode_leader_mappings = 1
```

Documentation has also been updated to match updated settings.