Add select_if() to select based on predicate

[lionel-]

This function would be analogous to summarise_if() and mutate_if().

One way to distinguish responsibilities between select() and select_if() is that the former operates on metadata such as the column names, while the latter operates on the columns themselves, and thus requires local sources.

So the custom functions suggested in #1229 would belong to select() because they operate on the column names. And more general predicates based on column types (also metadata) compatible with DB sources could be added to select().

Closes #497, closes #1569

[hadley]

I think it maybe should mimic mutate_if() and summarise_if() and always return grouped values. (It'll do that now but you'll get a message because of the default select() behaviour)

[codecov-io]

Current coverage is 20.01%

Merging #1880 into master will decrease coverage by <.01%

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[lionel-]

Here you go

[hadley]

Thanks!