Display of data that is "almost equal" between rows

[krlmlr]

• For numbers: e.g. 100.001, 100.002, 100.003, https://github.com/r-lib/pillar/issues/97#issuecomment-363699335
• For strings: e.g. "abcde", "abcdf", "abdfe", https://github.com/r-lib/pillar/issues/97#issuecomment-363699335, https://github.com/r-lib/pillar/issues/101

Should we implement a class that applies subtle styling to the parts of strings/numbers that are the same across all columns, and that sets the minimum width accordingly? Should we even move common information to the header?

[krlmlr]

This could become an extension to num() and char().