introduce the BLOCK_LIBEXEC env variable

On runtime, i3blocks now sets a BLOCK_LIBEXEC variable to the
environment. It is introduces for several reasons:

  * Factorize the configuration file (e.g. command=$BLOCK_LIBEXEC/foo)
  * It might help scripts to locate themselves, to load library files
    (e.g. source $BLOCK_LIBEXEC/ip.sh)
  * Easily switch/test different bench of scripts

The priorities are the following (higher first):

  1. The value of this <directory> argument
  2. Compile-time value: /usr/libexec/i3blocks or /usr/local/libexec/i3blocks
add a contrib/ directory

The new contrib/ directory is meant to include some well done and useful
scripts. It is scripts is minimal and generic enough, it might be a good
candidate to be used in the default status bar.

As a first candidate, migrate the "mediaplayer" script.
contrib: (mediaplayer) add cmus support

Add support for cmus by using the cmus-remote utility to query metadata.

https://cmus.github.io/

Closes #20
contrib: (openvpn) new script

Closes #18

[vd: fix typo i3block -> i3blocks]