Add read filter option

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

Example:
```go
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.