Make xsel preferred over xclip.

[modax]

It seems to be less buggy.
See #116 (comment)

Closes #116

[chocolateboy]

As mentioned here, xclip isn't "buggy" if used correctly.

Demoting it is a sticking plaster on platforms which have both, and doesn't fix #116 on platforms which only have xclip.

In addition, xsel launches a process in the background (i.e. a daemon) and provides no way to disable that behavior which works in a pipeline, whereas xclip -filter doesn't have any such surprises/side effects.

xsel

$ echo OK | xsel --clipboard --input

$ pgrep xsel > /dev/null && echo "running"
running

$ echo OK | xsel --clipboard --input --nodetach
<hangs>

xclip

$ echo OK | xclip -selection clipboard -filter
OK

$ pgrep xclip || echo "not running"
not running