Add command line option to serve gzip files

[yuri-karadzhov]

It would be helpful to have a command line option which do the same as { gzip: true }

[doot0]

Running static -h lists --gzip as the argument for this.

However, it doesn't seem to actually do anything, at least for me.

[voltrevo]

Works for me, except you need to have the uncompressed file available too:
#143

(As mentioned on the issue, you can put an empty file in for the uncompressed version, which is also an easy way to confirm the gzip mode is actually working.)