Add -w workspace and -d directory flags to plan/apply comments

[lkysow]

Now you can specify which workspace and directory to plan/apply in via

atlantis plan -d dir/child -w staging

Also, any additional options to proxy to terraform must be specified after a --:

atlantis plan -w staging -- -target=resource

Fixes #12

[codecov]

Codecov Report

Merging #14 into master will increase coverage by <.01%.
The diff coverage is 70.78%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   63.77%   63.78%   +<.01%     
==========================================
  Files          38       38              
  Lines        1778     1814      +36     
==========================================
+ Hits         1134     1157      +23     
- Misses        593      604      +11     
- Partials       51       53       +2

Impacted Files	Coverage Δ
server/events/apply_executor.go	0% <0%> (ø)	⬆️
server/events/project_pre_execute.go	96.61% <100%> (+0.53%)	⬆️
server/events/plan_executor.go	89.47% <100%> (+1.47%)	⬆️
server/events/event_parser.go	97.25% <89.47%> (-1.01%)	⬇️

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