Riffle v2.0.0-alpha.0

• Do not build with Node.js 12 with Actions. See #93.
• Remove unused dependencies from tests. Closes #102.
• Delete old README. Closes #101.
• Add chicklets and links to README.md. See #101.
• Caret pin Proof. See #101.
• Build with only Node.js 12 on Travis CI. See #101.
• Install Istanbul and Proof at Travis CI. See #101.
• Convert README.md to Markdown. See #101.
• Return array slices. Closes #103.
• Iterate over an empty tree. See #102.
• Update .gitignore and .npmignore. See #101.
• Build with Node.js 10 on Travis CI. Closes #98.
• Do not build with Node.js 8 on Travis CI. Closes #97.
• Do not build with Node.js 6 on Travis CI. Closes #96.
• Do not build with Node.js 4 on Travis CI. Closes #95.
• Build with Actions, ship coverage to Codecov. Closes #93. Closes #94.
• Build with Node.js 12 on Travis CI. Closes #92.
• Upgrade rimraf to 3.0.2. Closes #91.
• Update LICENSE for 2020. Closes #90.
• Install with NPM 5 on Travis CI.
• Use build utilities in PATH.
• Use shorthand GitHub repository URL.
• Update LICENSE date range for 2018.
• Do not build with Node.js 0.12 on Travis CI.
• Do not build with Node.js 0.10 on Travis CI.
• Upgrade rimraf to 2.6.2.
• Upgrade proof to 3.0.x.
• Build with Node.js 8 on Travis CI.
• Do not build with Node.js 5 on Travis CI.
• Add Wiseguy to .gitignore.
• Plain text README.
• Upgrade rimraf to 2.6.1.
• Upgrade Proof to 1.2.x.
• Update LICENSE for 2017.
• Get rid of test sub-directory.
• Tidy package.json.
• Build with latest Node.js 6 on Travis CI.
• Pin dependencies to minor version.

Closes #104.