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
Fix `uname` calls with `-o` parameter, as this is not BSD compatible.
As per @Eriner, use `uname -a` instead as a "catch-all" parameter.
Fixes#75, Closes#79