add generic options flag to gapic script

[noahdietz]

• add --options to gapic.sh
• add --python_gapic_opt=$PLUGIN_OPTIONS to Docker wrapper's protoc invocation

Note: ^ this requires an image build to take effect

Note: cross tested the script using an updated gapic-generator-go image, which worked

[codecov]

Codecov Report

Merging #66 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #66   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          17     17           
  Lines         698    698           
  Branches      139    139           
=====================================
  Hits          698    698

---

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[codecov]

Codecov Report

Merging #66 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #66   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          17     17           
  Lines         698    698           
  Branches      139    139           
=====================================
  Hits          698    698

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dd1b056...54251fd. Read the comment docs.