New Feature: Search data-filterTableKeywords as well as text

[tobya]

I had the requirement to filter based on extra text so I implemented code where the data-filterTableKeywords attribute on any tags is also checked for search string.

• existing code modified to use a function isMatch for all matching code which was repeated in several places
• added check of data-filterTableKeywords attribute on each match.
• added new example page filtertable-datatag.html and added to index.

Obviously i have only updated jquery.filtertable.js and not jquery.filtertable.min.js

I also wondered about adding a new config option to allow user to specify name of data attribute. Can look to do that if this pull request is accepted.

This is different to pullrequest #17 as it adds extra keywords to be searched on also rather than searching on a differnt value.

[tobya]

@sunnywalker Is there some changes I should make to this pull request for it to be merged?

[tobya]

@sunnywalker I have been using this change quite well on my own systems, is this something you are willing to merge in do you think?