Allow setting default selections in TerminalMenus

[christopher-dG]

This doesn't change any behaviour without opting into it with the default keyword, but allows you to set the initial cursor position (for RadioMenu) or checked options (MultiSelectMenu) when request is called.

Example (the menus are what appear without any input):

julia> m = RadioMenu(["a", "b", "c", "d", "e"]; default=3);

julia> request(m)
   a
   b
 > c
   d
   e
3

julia> m = MultiSelectMenu(["a", "b", "c", "d", "e"]; default=[2, 4]);

julia> request(m)
[press: d=done, a=all, n=none]
 > [ ] a
   [X] b
   [ ] c
   [X] d
   [ ] e
Set([4, 2])

[KronosTheLate]

Note that this is implemented. Example below:

julia> import REPL; using REPL.TerminalMenus

julia> menu = MultiSelectMenu(["Op 1", "Op 2", "Op 3"], selected=(2, 3));

julia> request(menu)
[press: Enter=toggle, a=all, n=none, d=done, q=abort]
 > [ ] Op 1
   [X] Op 2
   [X] Op 3