Fix cache subcommand and add tests

Treat the cache subcommand differently from --no-config:
For --no-config, insert args from selected environment variables
For cache, don't insert args

tests/examples/cache_source/syntaxes/c.sublime-syntax

@@ -0,0 +1,10 @@
+%YAML 1.2
+---
+name: C
+file_extensions: [c, h]
+scope: source.c
+
+contexts:
+  main:
+    - match: \b(if|else|for|while)\b
+      scope: keyword.control.c