This commit give aerc the ability to read commands through a FIFO
(named pipe). A command line flag is added (`-f`) which specifies the
path of the FIFO to read commands from. If no -f option is specified,
aerc doesn't read from any FIFO.
Again, please get rid of the FIFO, we already have a socket.
There's no need for two control handles.
The socket code is in lib/socket.go
So far it's pretty simple, if you connect and send a message with mailto:$url
it'll start the mailto handler.
We could introduce a new verb, say execute, again delimited from the command with
":" so that the parsing stays easy.
By that time, you'd also want to respond with either result: ok or result: error
depending on the outcome I guess, so we might need to tweak the mailto response
a bit to keep consistent.