Disable LP_ENABLE_SHORTEN_PATH inside Midnight Commander (#288)

The path was corrupted inside Midnight Commander. So we now disable the
'SHORTEN_PATH' feature (to force Liquid Prompt to use the standard shell
symbols to show the directory) when we detect we are launched from mc
(the existence of the MC_SID environment variable).
https://github.com/nojhan/liquidprompt/issues/288