Add option for smart column alignment based on decimal point

[tleonhardt]

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Competing modules such as tabulate are smart about column alignment by default. tabulate detects columns which contain only numbers, and aligns them by a decimal point (or flushes them to the right if they appear to be integers).

This is a highly desirable behavior and we should emulate it or at least provide an option to allow a user to choose it in addition to the existing column alignment options.