Add support to the Link class for the RFC 5988 attributes

[jstano]

Add direct support for the following attributes:
anchor, hreflang, media, title, type using the standard with builder style method

In order to support link extensions, we could do something like
withExtension(String paramName, String paramValue)

I will submit a pull request with changes to support this enhancement.

[dschulten]

Note that some param names may be repeated, others not.
The rev attribute is deprecated by rfc5988, it should not be a fixed attribute IMHO, could still be added as extension param.
The media attribute is also not immediately useful, it is about device-specific representations. Could also be added as extension, if needed.

Best regards,
Dietrich