Add command line option -s to monitor specific selections, add support for secondary selection

[andreblanke]

This pull request introduces changes discussed in #2. The selections to be monitored by clipnotify may now be specified via clipnotify -s clipboard,primary,secondary. If the -s option is not present all selections are monitored.

In addition to that I added support for the secondary selection for completeness, even though it is barely ever used.

[madprops]

Why isn't this merged?

[cdown]

Thanks, I've attributed you in a version of this patch with backwards compatibility fixes in master at 094dd7e20a06eba80c2e6f27dee775106e0eeca9.