Back-compatible custom key manglers

[wlandau]

So far, this PR is just a sketch of https://github.com/richfitz/storr/pull/89#discussion_r230660424. It came together quicker than I predicted. @richfitz, if you agree with implementation, then I will write tests.

[codecov-io]

Codecov Report

Merging #90 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   99.91%   99.91%   +<.01%     
==========================================
  Files          15       15              
  Lines        1179     1236      +57     
==========================================
+ Hits         1178     1235      +57     
  Misses          1        1

Impacted Files	Coverage Δ
R/driver_remote.R	100% <100%> (ø)	:arrow_up:
R/utils.R	100% <100%> (ø)	:arrow_up:
R/driver_rds.R	100% <100%> (ø)	:arrow_up:

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[wlandau]

This morning, I added bug fixes, tests, and documentation. I think this PR is ready for an initial review.

[richfitz]

Just to let you know - this is absoutely on my radar, but this week has been busy. I will look at this asap

[wlandau]

Thanks, Rich. FYI I will be on vacation from Nov 15 to 27, so I do not expect this work to be fast.

[wlandau]

@richfitz, what are your current thoughts on custom key manglers? Do you still this PR's approach to #88 is a good idea?