Add multiple bundles support

[nikolay-borzov]

• Replace expandGlob by getBundles that expands code glob into bundles list
• Handle multiple bundles using Promise.all
• Update generateHtml to handle multiple bundles. Use ejs to generate HTML content
• Move common functions to helpers object
• Replace _ usings by native implementations
• Add more JSDoc comments
• Update template
  • Generate webtreemap data on CLI side
  • Make font monospaced (for better readability)
  • Add bundle select where there is more than one bundle
• Remove underscore from dependencies
• Update temp, chai, eslint and mocha dependencies
• [Tests] Add some CLI tests
• [Test] Wire-up test coverage reporting
• [Travis] Replace node version 4 by 10 and 11
• [ESLint] Enable ES2018 syntax support (Promise, const, spread operator)

Resolves #25

[danvk]

@nikolay-borzov how strongly do you feel about keeping the monospace font? Here's a side-by-side with sans serif. You can see a lot more text with sans serif and, at least to me, the text is more visually distinct from the boxes.

[nikolay-borzov]

You can return sans-serif. monospaced seemed more readable to me