Support the --slow flag

[kevinburke]

From the mocha --help output:

    -s, --slow <ms>                         "slow" test threshold in milliseconds [75]

I'd like to set this to "2", but it looks like gulp-mocha only passes through a set number of options to the underlying mocha binary.

[spacem]

did you try it? I think this supports all the options. From the readme: "Options are passed directly to the mocha binary, so you can use any its command-line options"

[sindresorhus]

Yup, supports all the options.