Rewrite of scrub-themes script

[rgroves]

Script now adds the following to a theme's front matter:

• Add a stale flag if no commits in past year
• Add github branch (if missing)

Added scrub-themes to scripts in package.json.

Fixes #42

Notes on testing:
I did some manual testing using the following test scenarios:

Test 1: Theme's front matter has an existing github_branch key specified
Expected result: should not modify file
File used: content/theme/stackbit-theme-vanilla-gatsby.md

Test 2: Theme's front matter does not have a github_branch key specified
Expected result: should add github_branch: master
File used: content/theme/stackbit-theme-ampersand.md (Note: at the time of my testing master was this repo's default branch)

Test 3: Theme's front matter has no stale key
Expected result: should add stale: true
File used: content/theme/solar-theme-jekyll.md (Note: at the time of my testing this was a stale repo)

Test 4: Theme's front matter has an existing old stale key set
Expected result: should remove the stale: false entry and add an entry for stale: true
Manual setup: add stale: false to file before testing
File used: content/theme/pelican-smoothie.md (Note: at the time of my testing this was a stale repo)

Test 5: Theme's front matter has an existing stale key set
Expected result: Should remove the stale: true entry and add an entry for stale: false
Manual setup: add stale: true to file before testing
File used: content/theme/stackbit-theme-archetype.md (Note: at the time of my testing this was not a stale repo)

Note: For tests 3-5 a github_branch is also added because those theme files lacked an explicit github_branch key in the front matter.

All of the above tests were performed and passed as per the expected results before I submitted the PR

Here's what you can use to replicate my testing if you'd like to before merging:

# reset theme files to checked in versions (if needed)
git checkout -- content/theme/*

# Test 4 setup: add stale: false
sed -i '/title:/a stale: false' content/theme/pelican-smoothie.md

# Test 5 setup: add stale: true
sed -i '/title:/a stale: true' content/theme/stackbit-theme-archetype.md

# backup theme files for comparison after test
cp content/theme/stackbit-theme-vanilla-gatsby.md content/theme/stackbit-theme-vanilla-gatsby.md.bak
cp content/theme/stackbit-theme-ampersand.md content/theme/stackbit-theme-ampersand.md.bak
cp content/theme/solar-theme-jekyll.md content/theme/solar-theme-jekyll.md.bak
cp content/theme/pelican-smoothie.md content/theme/pelican-smoothie.md.bak
cp content/theme/stackbit-theme-archetype.md content/theme/stackbit-theme-archetype.md.bak

# Execute script under test
node scrub-themes.js

# Test 1 Result: Should be no differences
diff -s content/theme/stackbit-theme-vanilla-gatsby.md content/theme/stackbit-theme-vanilla-gatsby.md.bak

# Test 2 Result: Should add "github_branch: master"
diff -s content/theme/stackbit-theme-ampersand.md content/theme/stackbit-theme-ampersand.md.bak

# Test 3 Result: Should add `stale: true`
diff -s content/theme/solar-theme-jekyll.md content/theme/solar-theme-jekyll.md.bak

# Test 4 Result: Should remove the `stale: false` entry and add an entry for `stale: true`
diff -s content/theme/pelican-smoothie.md content/theme/pelican-smoothie.md.bak

# Test 5 Result: Should remove the `stale: true` entry and add an entry for `stale: false`
diff -s content/theme/stackbit-theme-archetype.md content/theme/stackbit-theme-archetype.md.bak

# Cleanup backup files
rm content/theme/stackbit-theme-vanilla-gatsby.md.bak
rm content/theme/stackbit-theme-ampersand.md.bak
rm content/theme/solar-theme-jekyll.md.bak
rm content/theme/pelican-smoothie.md.bak
rm content/theme/stackbit-theme-archetype.md.bak