Add .cols_align_decimal() for alignment of numbers to a decimal mark

[rich-iannone]

Aligning values by the decimal mark is desirable in lots of cases, especially where there are mixed numbers of decimal values shown in a column of numeric values. Not having this available means that numbers can be harder to scan through (even though they are, by default, right aligned in a column).

It can be challenging to do this right, but is possible. The particular challenges are:

• properly identifying the decimal mark
• decimal alignment with heterogeneously-formatted numbers (integers, decimal mark + no trailing digits, sci notn, etc.)