Allow format specificiers as Column format strings & deal with masked values.
Also refactored the string tests, and added regression test against formatting
of np.float32, etc.  Because masked values now are OK: [closes #1026]
Move description in changelog to 0.4.2 [skip ci]
Merge pull request #2898 from mhvk/column-format-string

Simplify table column format definition; fix printing of masked values for formatted columns