Add Prettier with tslint plugin and precommit hook

[agubler]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

Adds support for Prettier to for code style rules and formatting. Include precommit hook that runs prettier on all .ts files and an npm script that runs prettier against the codebase.

Related to dojo/meta#206

[codecov]

Codecov Report

Merging #156 into master will increase coverage by 0.09%.
The diff coverage is 84.18%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   84.98%   85.07%   +0.09%     
==========================================
  Files           1        1              
  Lines         506      516      +10     
  Branches      133      138       +5     
==========================================
+ Hits          430      439       +9     
  Misses         32       32              
- Partials       44       45       +1

Impacted Files	Coverage Δ
src/loader.ts	85.07% <84.18%> (+0.09%)	⬆️

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[agubler]

Add section to the README for Prettier usage & update the pull request template.