hem server host option correction in docs

In the docs the hem server option for host is written with only one
hyphen, which will not work. Optimist will parse -host x.x.x.x as
h:true, o:true, s:true and t: x.x.x.x (and test:x.x.x.x since there is
an alias for t)

The solution is to add a hyphen to the docs: --host or add an alias for
-h in the hem.coffee