Feature Request: Ignore lines not containing the delimiter character

[themightyoarfish]

A lot of the occasions where I use Tabularize are those where a set of lines with the character I want to align at are interleaved with ones that don't. These can be comments or other things. Consider e.g. this LaTeX code:

\toprule
A & B & C \\
\midrule
foo & bar & baz \\
oof & rab & zab

I want to align all lines at the & character, but this will be messed up by the ones which don't contain it.

[themightyoarfish]

Duplicate of #23