Add an option to do not switch to repl buffer on send command group

[illia-danko]

As a user I have the following setup:

• open a elixir code buffer on the left side;
• open a inf repl buffer on the right side;

Whenever I experiment with a function or a variable I evaluate it by either: inf-elixir-send-buffer once I've switched to file, inf-elixir-send-region or inf-elixir-send-line. That works, but I found tedious to switch back to the source code buffer every time after the evaluation.

Proposal:

• add a custom variable to make the switch to a repl buffer being optional.

[J3RN]

Fixed by #16