Add read filter option

[ndealmeida]

I would like to use the read command with a filter option filter rows based a cell value using bigtable.ValueFilter.

Example:

if filter := parsed["filter"]; filter != "" {
	filters = append(filters, bigtable.ValueFilter(filter))
}

Usage:
cbt read user-table prefix=user columns=login filter="^active.*" count=1

This would be quite useful for analysis and troubleshooting purposes.