Add -m option to command line

The -m option set the default system path for finding libraries
for that invocation of Janet.

CHANGELOG.md

@@ -2,6 +2,8 @@
 All notable changes to this project will be documented in this file.
 
 ## 0.4.0 - ??
+- Add `-m` flag to main client to allow specifying where to load
+  system modules from.
 - Add `-c` flag to main client to allow compiling Janet modules to images.
 - Add `string/format` and `buffer/format`.
 - Remove `string/pretty` and `string/number`.

src/mainclient/init.janet

@@ -22,6 +22,7 @@
   -r : Enter the repl after running all scripts
   -p : Keep on executing if there is a top level error (persistent)
   -q : Hide prompt, logo, and repl output (quiet)
+  -m syspath : Set system path for loading global modules
   -c source output : Compile janet source code into an image
   -l path : Execute code in a file before running the main script
   -- : Stop handling options`)
@@ -32,6 +33,7 @@
      "r" (fn [&] (set *should-repl* true) 1)
      "p" (fn [&] (set *exit-on-error* false) 1)
      "q" (fn [&] (set *quiet* true) 1)
+     "m" (fn [i &] (set module/*syspath* (get process/args (+ i 1))) 2)
      "c" (fn [i &]
            (def e (require (get process/args (+ i 1))))
            (spit (get process/args (+ i 2)) (make-image e))