setup environment variables

i3blocks will set the environment variables BLOCK_* before forking a
block command. Actually, BLOCK_NAME and BLOCK_INSTANCE are provided.

Here's a example configuration file using the environment:

    [test1]
    command=echo NAME=$BLOCK_NAME INSTANCE=$BLOCK_INSTANCE
    interval=1

    [test2]
    command=echo NAME=$BLOCK_NAME INSTANCE=$BLOCK_INSTANCE
    instance=foobar
    interval=1