Addresses bash-it/bash-it#1632
And use `_log_warning`.
Alsö, code style cleanup: quote things, handle unbound parameters, &c.
Alsö alsö, short-circuit if not installed or inadequate _Bash_ version.
The original patch (659ecd0388)
unaliases the percol alias, however, does not validate if the alias
is already defined. This leads to the following message that is shown
everytime a new bash session is spawned where this variable is not
defined when the percol plugin is enabled.
bash: unalias: zz: not found
Without the `function` keyword, the file did not load when using Bash
v3.x. With the added `function` keyword, the file can be parsed and the
error message about having to install Bash v4 is shown.
Closes#881
John D Pell
John McBride
Nils Winkler
Adam Wallis
codefalling