This is configurable via an environment variable rather than a command line
option, as I'm not expecting many people really need to change this thing.
I profiled the change from 1 -> 0.5 seconds on an old P4, and saw pretty much
no difference in CPU usage, whether there were new clipboard entries or not.
Closes#12.
This avoids getting a bad array subscript, and polluting the menu.
Thanks to @kaihendry for noticing this issue and suggesting a fix. This commit
is a modified version of his suggestion, using [:blank:] instead of [:space:].
Closes#7.