Add readline-style keybindings

This commit adds a common subset of readline style keybindings, as requested in issue #216 and issue #460, which should satisfy most users used to the Unix command line (or Unix clients like `irssi`). It implements the following new bindings:

Profanity Default | Readline Style Equivalent    | Readline Command
----------------- | ---------------------------- | -------------------
<kbd>Del</kbd>    | <kbd>Ctrl</kbd>+<kbd>D</kbd> | `delete-char`
<kbd>←</kbd>      | <kbd>Ctrl</kbd>+<kbd>B</kbd> | `backward-char`
<kbd>→</kbd>      | <kbd>Ctrl</kbd>+<kbd>F</kdb> | `forward-char`
<kbd>↑</kbd>      | <kbd>Ctrl</kbd>+<kbd>P</kdb> | `previous-history`
<kbd>↓</kbd>      | <kbd>Ctrl</kbd>+<kbd>N</kdb> | `next-history`
<kbd>Home</kbd>   | <kbd>Ctrl</kbd>+<kbd>A</kdb> | `beginning-of-line`
<kbd>End</kbd>    | <kbd>Ctrl</kbd>+<kbd>E</kdb> | `end-of-line`
                  | <kbd>Ctrl</kbd>+<kbd>U</kdb> | `unix-line-discard`

Very experienced readline users may also miss <kbd>Meta</kbd>+<kbd>B</kbd> and <kbd>Meta</kbd>+<kbd>F</kbd> for `backward-word` and `forward-word`, and <kbd>Ctrl</kbd>+<kbd>K</kbd> for `kill-line`, but, I think the above will be enough to make most Unix junkies happy.