Support less-aggressive character encoding

[twostraws]

Right now swift-html-entities aggressively encodes everything, even when it's perfectly acceptable inside HTML. For example, line breaks become 
.

HE, the library this is based on, has an escape() function that "takes a string of text and escapes it for use in text contexts in XML or HTML documents… only the following characters are escaped: &, <, >, ", ', and `." PHP has a htmlspecialchars function that does something similar, although it ignores backticks.

Is it possible to get something similar here?

[youming-lin]

@twostraws Based on my observation, by default he only escapes &, <, >, ", ', and ` in the ASCII space. Are these six characters enough?

[twostraws]

Yes, I think that ought to be safe for general rendering of content.

[youming-lin]

Code has been updated to skip the non-printing ASCII characters when escaping in #11.

Along with additional changes to the package to bring it more in line with he's encode options, a new version 3.0.0 release has been tagged.