feat: add support for monorepos/releasing from alternate folders

[bcoe]

This PR adds support for releasing libraries from a folder other than a repository's root:

You can now set the option --path which represents the path that package lives in, e.g., --path packages/gcf-utils.

You can also set the flag --monorepo-tags, which will prepend the library name to release branches and tags (this allows you to release multiple libraries from one repository (see: bcoe/test-release-please#43).

[codecov]

Codecov Report

Merging #501 into master will decrease coverage by 0.00%.
The diff coverage is 67.50%.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
- Coverage   81.28%   81.27%   -0.01%     
==========================================
  Files          41       41              
  Lines        4745     4829      +84     
  Branches      393      378      -15     
==========================================
+ Hits         3857     3925      +68     
- Misses        887      903      +16     
  Partials        1        1              

Impacted Files	Coverage Δ
src/releasers/java-auth-yoshi.ts	22.92% <0.00%> (-0.60%)	⬇️
src/releasers/python.ts	37.19% <0.00%> (-1.28%)	⬇️
src/releasers/ruby-yoshi.ts	28.49% <0.00%> (+0.31%)	⬆️
src/releasers/ruby.ts	36.52% <0.00%> (-1.32%)	⬇️
src/releasers/terraform-module.ts	33.00% <0.00%> (-1.38%)	⬇️
src/github-release.ts	93.78% <87.50%> (-0.77%)	⬇️
src/github.ts	76.33% <100.00%> (+0.12%)	⬆️
src/release-pr.ts	90.36% <100.00%> (+1.22%)	⬆️
src/releasers/java-bom.ts	93.91% <100.00%> (+0.02%)	⬆️
src/releasers/java-yoshi.ts	88.31% <100.00%> (+0.33%)	⬆️
... and 3 more

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