Enhancement request: Provide a way to import from additional directories

[wasamasa]

It seems that eisl only allows having one library directory. When I install it system-wide, it relies on me setting EASY_ISLISP to import libraries from it (see #199 for an idea how to do better). I see several ways of extending this:

• Offer an additional environment variable containing colon-separated extra paths to search
• Offer CLI flags for prepending/appending extra paths to search
• Offer a Lisp variable or function for adding extra paths (and some way to use it via CLI flags)

[sasagawa888]

I would like to keep the Easy-ISLisp mechanism as simple as possible. I will leave it as is.
see POLICY.md

[wasamasa]

Alright then. I'll instead use the -l flags.