so it's not a template anymore and can be under version control, instead
of being manually updated by the user, as currently is the case with
`~/.zlogin` and `${ZIM_HOME}/templates/zlogin`.
Consider the `zcompdump_file` variable, that could be defined in
`.zimrc`, when cleaning the cache.
A custom zcompdump_file would not be cleaned from the cache without this
correction.
Also remove *.zwc.old files from cache, as zrecompile also generates
these.
This change allow arbitrary Zim location path by setting a new
environment variable, ZIM_HOME. If the user does not set it, Zim falls
back to the old "${ZDOTDIR:-${HOME}}/.zim" location.
Closes#203
