Adds custom control mapping

[Lynesth]

Allow mapping custom keys to callables.
It works per menu so you can set custom controls for shortcuts for example.

$myCallback = function(CliMenu $menu) {
    // Do something
}

$menu = (new CliMenuBuilder)
    ->addItem('List of [C]lients', $myCallback)
    ->build();

// Now, pressing Uppercase C (it's case sensitive) will call $myCallback
$menu->addCustomMapping("C", $myCallback);

$menu->open();

[codecov-io]

Codecov Report

Merging #87 into master will decrease coverage by 1.22%.
The diff coverage is 8.33%.

@@             Coverage Diff              @@
##             master      #87      +/-   ##
============================================
- Coverage     96.47%   95.24%   -1.23%     
- Complexity      278      284       +6     
============================================
  Files            22       22              
  Lines           879      884       +5     
============================================
- Hits            848      842       -6     
- Misses           31       42      +11

Impacted Files	Coverage Δ	Complexity Δ
src/CliMenu.php	84.53% <8.33%> (-5.82%)	66 <5> (+6)

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[AydinHassan]

Nice feature and example - I like.