Add a reason keyword arg to ParseError#initialize

[sampersand]

This adds a reason: keyword argument to ParseError#initialize for ergonomics and ease-of-use.

Before:

OptParse.acceptEncodingdo |arg|
Encoding.findargrescueArgumentErrorerr=OptionParser::InvalidArgument.newerr.reason="not a valid encoding"raiseerrend

Now:

OptParse.acceptEncodingdo |arg|
Encoding.findargrescueArgumentErrorraiseOptParse::InvalidArgument.new(reason: 'not a valid encoding')end