Custom mapping for verb input

First of all thanks for such an awesome tool.

I have the following line in my `.vim/vimrc`, which saves me having to type `SHIFT` to enter a command:

``` vimrc
nnoremap ; :
vnoremap ; :
```

Is is possible to have a similar mapping in `broot`, allowing me to enter verbs starting with `;` instead of `:` (in effect, I would be typing `;quit` for example)? I think this would not negatively impact the normal filter operation, since `;` is not a valid filename.

I have played around with verbs and invocations but am pretty sure that `broot` currently only allows mapping keys like functions keys and keys with modifiers.

Start by tracing broot's verb and invocation handling, then inspect how key mappings are parsed and how `;` is treated by the normal filter operation. Implement support for entering verbs with a configurable `;` prefix without breaking filtering, and verify that an example such as `;quit` invokes the quit verb.

Written by the indexing model from the issue text.