Implemented passing of headers to "attack" using a -H argument. Fix for issue #32

Implemented passing of headers to the bees when attacking.

The format for a single header is as follows:
bees attack -H "key:value" ...

Multiple headers can be ";" seperated, e.g.
bees attack -H "key1:value1;key2:value2" ...

Regards,
@roylines and @jamesbloomer