fix numeric sorting in `qiime feature-table tabulate-seqs`

[gregcaporaso]

Numeric columns in tabulate-seqs sort alphabetically, not numerically. They should sort numerically.

forum x-ref

Also referenced in an issue posted by @colinbrislawn in #214.

[gregcaporaso]

@Macabe222 - would you mind working on this one?

[Macabe222]

@gregcaporaso yes I will get started now!