Added a delimiter-option

Because a hashsum file can have different layouts corresponding to the separator between the hashsum and the filename (e.g. one space or two spaces), I added an option to the gulp-hashsum, where you can define which delimiter to use. The default is set to two spaces, so nothing changes for those who are using it like this. In my company we only have one space as delimiter and now we can simply set this via the delimter-option.