Renamed Ronin::UI::Shell#call to #run.
Whitespace--
Default @input_handler to the run() method.
Simplified UI::Shell by just yielding the input line.

* Removed the #write and #<< methods.
Split Ronin::UI::Shell into Ronin::UI::REPL.

* Ronin::UI::REPL is meant for quick REPLs, where input is handled
  and a result is printed.
* Ronin::UI::Shell is meant for designing custom shells with commands.