Strict and complete tests of sorting across test suites

[edelooff]

N.B. This PR builds on changes introduced in #17 (so merge that one first for history's sake)

This set of changes replaces the existing test-tables with altered ones that exercise and verify the different sorting methods in timbles:

• Lexicographical case-sensitive sort
• Lexicographical case-insensitive sort
• Lexicographical sort of integers
• Natural sort of integers
• Natural sort of reals (percentages)

It also provides test for the newly added valueTransform and textTransform which allow the same sorting tests to be used across existing tables and generated tables, providing tests for the feature described in #16 and added in #17.

It also adds code to correctly use the data-value attribute even if its content is falsy. If it's present, we shall use it. This resolves #13.

Finally this PR simplifies tests by getting rid of the specific test suite for ID-free operation (this is no longer a special feature and present in the pre-existing table suite), and getting rid of the data-filter suite, as filters are already exercised in all suites that test generated tables. This should make future work on #12 a lot easier.

Merry Christmas :santa:

[jennschiffer]

this looks great - add an example of the value transform to the readme and then I'll merge!

[edelooff]

Added to the branch from PR #17 and rebased the whole thing. Both the text- and the valueTransform now have a simple example

[edelooff]

Example added (as per previous post) and there seem to be no other obvious warts, so I'm merging this.