Change all whoami calls for $USER variable

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.
Change all pwd calls for $PWD variable

This avoids spawning additional processes as $PWD **always** contains
the current working directory.
Change all hostname calls for $HOST or $SHORT_HOST
Don't require toilet in ragekill plugin

In systems without root permission you can't install toilet. This change
lets you use the flip script included within the ragekill plugin if
toilet is not found, to flip the process name instead.

It also changes the naming of the plugin in a couple more places which
weren't changed at the time, and redoes some logic paths to optimize
and clean up the plugin.
Minor fixes in INSTALL.md in ragekill plugin
Include batcharge.py into batcharge function

This prevents a not found error when the batcharge.py script cannot be
found, which happens whenever we checkout a point in time where the
batcharge plugin is not there.

By including it directly into the batcharge plugin function we achieve
both the disappearance of the error mentioned above and ensure a fully
functioning batcharge plugin (as long as the batcharge function is
still there).

An alternative solution was to put a check for the batcharge.py script
inside the batcharge function, before calling it with python. This,
however, wouldn't mantain the plugin's functionality.

A desirable future change will be to translate the python script into
zsh code, so as to avoid running a separate process in every call of
the batcharge function and remove the necessity of a python binary.
Merge branch 'use-special-vars' into dev
Merge branch 'ssh-agent-force-bourne-syntax' into dev