Add command to get/set options

[pawamoy]

I was thinking of something like:

# get global option
aria2p getopt max-concurrent-downloads

# get downloads options
aria2p getopt dir 390c3d203c46ae0a

# set global option
aria2p setopt max-concurrent-downloads=10

# increment/decrement global integer option
aria2p setopt max-concurrent-downloads=+4
# or +=4? maybe support both. Should be easy as this is a positional argument

# set downloads options
aria2p setopt allow-overwrite=true 390c3d203c46ae0a 2089b05ecca3d829

We should be able to combine this with the "select" and/or "filter" flags of #22.

This issue also requires #14 to be implemented.