Better REPL-choosing Logic

This release includes a fix for #7. The new logic for REPL-choosing it illustrated in a new wiki page.

Also included is an inf-elixir-set-repl command which prompts the user for a REPL to target for inf-elixir-send-* commands for the current buffer. The prompt includes a "Create REPL" option in case you'd like to target a brand new REPL.

Incremental feature update

• Add inf-elixir-run-cmd which receives a command as a string and opens a new Elixir REPL and buffer running that command.
• Add all "sent" commands (e.g. commands run with inf-elixir-send-line) to the shell's history.

Initial MELPA Release

This includes the core functionality of inf-elixir with the ability to open Elixir REPLs, interact with them in their buffer, and send snippets from other buffers.