Filtering table field support ?

[heu]

This is missing feature as a note in README, but after take a look in code I've found that it is a trivial change as :
sync.go:197

func (r *River) makeInsertReqData(...) {
...
    if name != "-" {
    req.Data[name] = r.makeReqColumnData(&c, values[i])
    } 
...

and similar for makeUpdateReqData

By this way I can filter table field in rule's field config as:

[[rule]]

    [rule.field]
    password = "-"

So my question is: Does it has any risk?

[siddontang]

@heu

- is a common ignoring way in go, e.g, for json codec, so I think it is ok, we cannot create a column naming - :-).

But we should change some places, and write documents in README for this feature.