added missing word in README.

Supply these global flags to subcommands. Closes #69

closes #54

Changes:
 - `browse` and `compare` always open HTTPS
 - the `-p` flag for these commands has no effect anymore
 - updated `browse` URL for wiki pages
 - cherry-picking a URL doesn't add a new remote as private

references a01b256f

closes #63

New implementation allows for any number of chained commands.
The main command executes between "before" and "after" execution.

Commands can be simple strings, argument arrays or procs.

closes #64

A git remote can be configured with multiple "url" values.
Read them all in and pick the first GitHub one.

Closes #59

Hub::Context methods were public, now they are private. Ensures that
`Hub::Commands.respond_to?` doesn't return true for any of its methods.

Closes #62

hub would erroneously display the help page instead of forwarding the
arguments to git.

Bonus: make test/fakebin/git POSIX shell compatible; was using bashisms.