adds support for minijasminenode's showColors option

@sindresorhus - Thanks for taking the time to create and share this gulp plugin! I like to have color output when I run tests in the terminal and thought maybe others might also. So, the changes in my commit: 

*  Add support for the `showColors` option for [minijasminenode's executespecs() function](https://github.com/juliemr/minijasminenode/blob/master/lib/index.js#L70)

*  Add documentation to your repo's README for the `showColors` option in the same format / convention you used for the other options

i.e. 

##### showColors

Type: `Boolean`
Default: `false`

Print colors to the terminal