Allow setting a custom server with --server URL

[rpls]

This is a simple patch for #221. Could probably use some more bells and whistles, e.g., a re.match to check the URL. But this works so far.

Closes #221

[kennyballou]

As an outsider wanting this patch to land, what's the usage? How do I actually specify my own URL for languagetool? Is it combined with another parameter I'm not seeing from the patch? Could you please add a bit to the README for how to use this?

[torik42]

If you just want to use this patch and merge yourself, you might want to look at #221.

If the intention is to have a more complete patch which will be merged soon, I have to say the following: The complete settings section of yalafi.shell in yalafi/shell/shell.py is messed up and needs to be cleaned. This patch indeed works but is not very clean in that it sets cmdline.server = 'lt'. Unfortunately, before I will come to that, I have some other things to fix. And fixing the interface and command line options might even require a new major release.

[kennyballou]

The instructions in #221 are sufficient for me if you want to merge this as is. Thank you.