Allow preparing of subMenus

[Lynesth]

This allows passing another CliMenuBuilder as the second argument of CliMenuBuilder::addSubMenu().

Why this request ? Because I think it gives more control for custom menus and I think it's easier to write more organized code that way.

Usage example:

$subMenu1 = (new CliMenuBuilder)
    ->setTitle('Some title')
    -> ... ; // No ->build() at the end !

$subMenu2 = (new CliMenuBuilder)
    ->setTitle('Some other title')
    -> ... ; // No ->build() at the end !

$mainMenu = (new CliMenuBuilder)
    ->setTitle('My awesome menu')
    ->addSubMenu('menu1', $subMenu1)
    ->addSubMenu('menu2', $subMenu2)
    ->build();

[codecov-io]

Codecov Report

Merging #85 into master will decrease coverage by 0.32%.
The diff coverage is 57.14%.

@@             Coverage Diff              @@
##             master      #85      +/-   ##
============================================
- Coverage     96.47%   96.14%   -0.33%     
- Complexity      278      279       +1     
============================================
  Files            22       22              
  Lines           879      883       +4     
============================================
+ Hits            848      849       +1     
- Misses           31       34       +3

Impacted Files	Coverage Δ	Complexity Δ
src/CliMenuBuilder.php	96.47% <57.14%> (-2.08%)	44 <2> (+1)

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[AydinHassan]

Seems reasonable enough to me. Thanks @Lynesth - I will add some tests and docs.