The docopt-go library is quite convenient, but it has been abandoned for
a while :(
Since we only use it for chasquid-util, this patch removes it and
replaces it with a custom small parser, that is a reasonable fit for the
required use cases.
The patch also adds a couple of tests to increase coverage.
NOTE: docopt-go accepted some undocumented behaviour, in particular the
use of "-a b" instead of "-a=b". The new parser does not, so some
user scripts may require updating.
I think this should be rare enough not to be worth the complexity of
adjusting the parser to allow it.
