Support dirs with spaces

[lkysow]

Allow running atlantis plan -d "dir with spaces" to target a directory
that contains spaces.

Fixes #423

[codecov]

Codecov Report

Merging #515 into master will decrease coverage by 0.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
- Coverage   72.04%   72.02%   -0.02%     
==========================================
  Files          62       62              
  Lines        4465     4469       +4     
==========================================
+ Hits         3217     3219       +2     
- Misses       1005     1006       +1     
- Partials      243      244       +1

Impacted Files	Coverage Δ
server/events/terraform/terraform_client.go	78.86% <ø> (ø)	⬆️
server/events/comment_parser.go	96.66% <71.42%> (-1.61%)	⬇️

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