Add support for application specific sound

Allows for setting the notification sound for pushover-weechat.
Clean up the formatting of title and message sent to Pushover

Splits signal_data and sends the sender name in title, while the actual
message gets sent in…message. (Crazy talk, I know.)
Updated readme for sound and interval

Added information for interval and sound options.
Markdown is a thing I should probably be familiar with

Hopefully this works
Merge pull request #3 from EricaJoy/master

Added app specific sound support and cleaned up formatting of the Pushover message