print out important snakemake args when invoking genome-grist --help

[clb21565]

genome-grist run [ <target 2>... ] [ snakemake args ]

i had to look around to find what the appropriate snakemake args were (-j and -p) as i could not quickly* figure out what they were

also, genome-grist --help prints the correct menu, but doing:
genome-grist run --help errors out for lack of a config file input:
(grist) genome-grist run --help
Usage: genome-grist run [OPTIONS] CONFIGFILE [SNAKEMAKE_ARGS]...

Error: Missing argument 'CONFIGFILE'.

thus, it might be more intuitive to change that

[ctb]

great idea! thank you!