accept command line arg for a custom header

[lanterndev]

This builds on @luk-'s work in #28 to allow specifying a custom header via the -H or --header command line argument.

A future change could allow for specifying more than one custom header via a --headers command line argument. It looks like optimist does not support recovering multiple passed-in -H arguments, so unfortunately emulating curl isn't an option (e.g. curl -H 'X-Foo: Bar' -H 'X-Baz: Qux' http://www.google.com/).

For now, this is definitely useful for e.g. adding an HSTS header, so I thought I'd send a pull request.

[andrefs]

+1

This would allow, for example, to force the Content-Type of the files being served.
Useful if you're exposing some kind of file storage where all the files have the same type.

[indexzero]

The problem with this implementation is that it doesn't support multiple values being passed to -H. We should support that option too.

[indexzero]

Closing. No follow-up from author.