Misc improvements

[dhui]

Various improvements include:

• only set -common_args option if there are addition init args
• fix typo
• wait for exiftool command to exit on Close()
• Run tests that call NewExiftool in parallel (for me, test runtimes dropped from 3-4s to ~1s)
• Run CI on push and pull requests

[codecov-commenter]

Codecov Report

Merging #41 (699bee9) into master (f55d3fd) will increase coverage by 1.25%.
The diff coverage is 76.19%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   88.19%   89.44%   +1.25%     
==========================================
  Files           2        2              
  Lines         144      180      +36     
==========================================
+ Hits          127      161      +34     
- Misses         11       13       +2     
  Partials        6        6              

Impacted Files	Coverage Δ
exiftool.go	83.33% <76.19%> (+3.10%)	⬆️
filemetadata.go	100.00% <0.00%> (ø)

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