Add support for configurable imports

[nikolap]

Re https://github.com/greglook/cljstyle/issues/31

[codecov-io]

Codecov Report

Merging #39 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   92.67%   92.69%   +0.01%     
==========================================
  Files          13       13              
  Lines        1515     1519       +4     
  Branches       44       44              
==========================================
+ Hits         1404     1408       +4     
  Misses         67       67              
  Partials       44       44

Impacted Files	Coverage Δ
src/cljstyle/config.clj	98.19% <100%> (+0.01%)	:arrow_up:
src/cljstyle/format/ns.clj	96.67% <100%> (+0.03%)	:arrow_up:

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[nikolap]

Tweaked this to instead of be configurable by vector/list, simply use a flag, :vector-for-imports?

Rationale is there isn't another commonly used valid option.

Let me know if there's anything you'd like done before this can get merged

[greglook]

Sorry for the delay; this needs a bit of updating to work with the latest configuration style now.