Added support to unicode and single quote char in css

[RioloGiuseppe]

In this pull-request:

• Changed the regex for reading the "content" value from css.
• Added support to single quotes ', together to double ".
• Added support to non-starting \f values eg. \a584 or \a584.
• Added support for unicode values eg. \ua584.

Solved issues:

• not working with materialdesignicons #32
• Some icons work, some icons do not... #24
• why (fa-spin it's not working #23
• Doesn't work with single quotes on CSS #20
• material-design-icons css parsing issue #17

[modos189]

Hi @NathanWalker can we expect this PR to be merged?

[NathanWalker]

@modos189 I'm thinking we should just build this directly into {N} 8.3 core as a new feature - it's always felt like should just be in core by default; any thoughts? if so I could add it as PR to core including the details included here and publish an alpha until 8.3 is ready that you could use as a built-in feature of core.

[modos189]

@NathanWalker Yes, I think that's a good idea