Support for Custom Attributes

[eddiekaiger]

Introduces a new enum case for Attribute that allows for custom attributes:

Attribute.custom(String, Any)

You can also use this with extensions, like so:

"Hello".withCustomAttribute(named: "Foo", value: 42)

Background:
The Apple Docs state the following with regards to attributed strings:

You can assign any attribute name-value pair you wish to a range of characters, in addition to the standard attributes.

Hence, it makes sense for SwiftyAttributes to support any attribute names, not just the standard ones created by Apple.

Solves #25.

[codecov-io]

Codecov Report

Merging #26 into master will increase coverage by 0.28%.
The diff coverage is 96.96%.

@@            Coverage Diff            @@
##           master     #26      +/-   ##
=========================================
+ Coverage   99.22%   99.5%   +0.28%     
=========================================
  Files          10      10              
  Lines         516     408     -108     
  Branches      121       0     -121     
=========================================
- Hits          512     406     -106     
  Misses          2       2              
+ Partials        2       0       -2