minor improvement: make mk*_or_fail chainable.
minor improvement: make mk*_or_fail chainable.

Rather than composing the filename in a variable, passing it to
mkfile_or_fail or mkdir_or_fail, then using that variable elsewhere,
you can now set the variable to (or return the value of) the output of
the called function, which will be the filename you requested.

starkandwayne/genesis,starkandwayne/genesis,starkandwayne/genesis