[#22/#26/#4]: Add unit tests, linting, and quiet CLI flag

[paulshryock]

Summary

Added Ava unit tests, and cleaned up some code along the way. As a result, the main classes are written better so that updating state and creating side-effects are in separate methods. Added linting as well.

Added Ava tests for the following classes:

• Bump
• Git
• Changelog
• WordPress
• utils/file
• utils/type

Changelog

Added

• Add unit tests. [Add unit tests #22]
• Add type util methods.
• Add quiet CLI flag.
• Add linting. [Add linting #26]

Changed

• Update file util methods.

Testing

• Run npm test and see that all tests pass

Test coverage

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	79.62	63.64	75	79.62
src	77.48	59.46	70.59	77.48
Bump.js	78.03	63.64	40	78.03	49-59,74-75,103-105,114-124,130-131
Changelog.js	84.71	60.71	80	84.71	...,88-98,114-115,121-122,148-150,166-168
Git.js	85.96	76.92	100	85.96	23-25,35-36,53-55
WordPress.js	66.03	45.45	75	66.03	...06-109,112-120,131-134,137-143,152-154
src/utils	96.88	85.71	100	96.88
file.js	96.23	83.33	100	96.23	49-50
type.js	100	100	100	100

Ticket

• Closes Add unit tests #22
• Closes Add linting #26
• Add CLI flag configuration #4

Checklist

• I have tested the code in this pull request.
• I have updated the Changelog.
• I have documented any new features or changes in the Readme.