grammar - allow a lone literal to search all columns

This commit allows a search using a literal to search all columns.
For example, searching for:

foobar

will search all columns of all rows for foobar. This is the
natural inclination of people using this text box, as evidenced by
[1], so this will make tablequeryjs simpler to use for people who
do not read help text about it.

[1] https://help.github.com/articles/rendering-csv-and-tsv-data