fix riml_include bug

Also, separate Riml.source_path from new path, Riml.include_path.

Riml.source_path and Riml.include_path now accept colon-separated
arguments, and can be set via environment variables RIML_SOURCE_PATH
and RIML_INCLUDE_PATH, respectively.

Change Riml.source_path set from command-line to -S flag, to mirror
Riml.include_path being set by -I flag.