that could be defined in `.zimrc`, when doing the zcompile in
the completion module and when cleaning the cache.
A custom `${zcompdump_file}` would not be zcompiled or cleaned from the
cache without this correction.
Also, use zrecompile instead of the custom logic to call zcompile.
This change allow arbitrary Zim location path by setting a new
environment variable, ZIM_HOME. If the user does not set it, Zim falls
back to the old "${ZDOTDIR:-${HOME}}/.zim" location.
Closes#203