Dev

[davidagold]

This PR continues integrating @johnmyleswhite 's work in jplyr_notes (and includes him in the copyright statement in the license) and introduces two major changes:

1. The functionality previously provided mutate is now provided by select, as in SQL. It would be nice to provide a shorthand for hcat(tbl, select(tbl, ...)). The jury is still out on what that should be.
2. We take the nuclear option suggested in #2 and completely remove what I've been referring to as the "one-off" interface (the macros @filter, @select, etc.). All manipulation goes through either @query or a new convenience macro @qcollect, which is just like @query except it automatically collects the resultant graph, saving a step for the user.

[codecov-io]

Current coverage is 57.38% (diff: 87.27%)

Merging #14 into master will decrease coverage by 4.95%

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