Add --config and --runtime options

Use GNU-style long options to avoid ambiguous, one-character options
in the spec, while still allowing the runtime to support one-character
options with packing.

Signed-off-by: W. Trevor King <wking@tremily.us>

runtime.md

@@ -8,7 +8,9 @@ It operates by default on the `config.json` and `runtime.json` in the current di
 
 Starts a container from a bundle directory. 
 
-* *Flags:* none.
+* *Options*
+  * *`--config <PATH>`* Override `config.json` with an alternative path.  The path may not support seeking (e.g. `/dev/fd/3`).
+  * *`--runtime <PATH>`* Override `runtime.json` with an alternative path.  The path may not support seeking (e.g. `/dev/fd/3`).
 * *Standard streams:* The runtime must attach its standard streams directly to the application process without inspection.
 * *Exit code:* The runtime must exit with the application process's exit code.