Add Feature: Per type newLine separator

[GusGold]

Allows providing an object to options.newLine to specify a file separator based on the file type. e.g. Joining js types with a semicolon and newline, and css with just a new line:

useref({
    newLine: {
        js: ';\n',
        css: '\n'
    }
})

• Preserves all existing behaviour/assumptions
• Extends fix for #241
• Maintains #249
• Implements variant of #248

[coveralls]

Coverage increased (+0.02%) to 98.204% when pulling 4cc0edc525ec967b82c5933a1e153aaa6b229c10 on GusGold:master into 5a632ab18d3938027e21b6c1ac4a3149749c33ec on jonkemp:master.

[GusGold]

Hey @jonkemp

Please take a look at this PR when you get a chance. Thanks.

[GusGold]

A friendly reminder to take a look at this PR @jonkemp

[jonkemp]

Thank you for your interest. I am maintaining this project but not actively developing it. Pull requests are still welcome but a pull request that implements a new feature is going to take some convincing to add. I am not currently convinced that there is enough interest in this feature to warrant adding it at this time. That said I am more than happy to leave it open for feedback. If there is sufficient interest in this PR from others, I will reconsider it then.

[GusGold]

Okay, no worries