Swift 4 and 3.2

[eddiekaiger]

The biggest change in the Swift 4 conversion is the removal of Attribute.Name in favor of a new enum NSAttributedStringKey. The enum is essentially identical to Attribute.Name with the exception of Attribute.Name.textColor corresponding to NSAttributedStringKey.foregroundColor. As a result, very few (if any) projects will experience breaking changes.

Also, attribute(_:at:effectiveRange:) has been renamed to swiftyAttribute(_:at:effectiveRange:) to disambiguate itself from the same method found in Foundation.

This PR enables SwiftyAttributes to be built with both Swift 3.2 and Swift 4.0 in Xcode 9.

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   99.82%   99.86%   +0.03%     
==========================================
  Files          21       19       -2     
  Lines        1175     1448     +273     
==========================================
+ Hits         1173     1446     +273     
  Misses          2        2