Add build config/scripts for distribution

[ktsn]

This PR solves #14 - it adds a build script that compiles all src/ files to .js or .css and put them into dist/ directory. Then the users of eagle.js does not need to prepare extra dependencies for build that eagle.js depends on. The build script is at build/build.js. We can execute it with npm run build or automatically execute before publishing to npm.

Sorry for the long description 😅 . I tried to keep simple to describe this patch but I need such the number of sentences finally.

Build script

The build script does the following things:

• Compile all .scss files in src/themes/ and output into dist/themes/ directory.
• Copy all src/themes/assets/ int dist/themes/assets/
  • I've move all images in themes/ into themes/assets/ for convenience. 🙂
• Compile/bundle all .js and .vue files into dist/eagle.js. Also generate dist/eagle.css that has style block contents in all .vue files.
  • Using rollup for bundle because it generates smallest code among all bundlers, AFAIK.

Extra things that this PR includes

• In package.json
  • Move vue from dependencies to peerDependencies
  • Because if eagle.js include vue as a dependency, it restricts the users what version of vue they can use. peerDependencies just notifies the users that eagle.js requires vue, so they can select the version they want to use.
  • Clean up dependencies that does not used by eagle.js
  • Add files field to only publish dist/ and src/ directories to npm.
• Rename theme files to let Sass compiler aware which file should be distributed. And move all images into themes/assets
  • I don't confident so much about themes. If anything is going wrong, please let me know.
• Modify a few styles in .vue files to scope because rollup-plugin-vue does not support scoped css yet. It will be supported since v2.4.

Possible effects of this change

• Moving vue from dependencies to peerDependencies is technically a breaking change because npm no longer install vue automatically if the users only specify eagle.js in their dependencies. But I think its effect is smaller.
• import Eagle from 'eagle.js' no longer load component styles since they are separated to dist/eagle.css. The users need to import eagle.js/dist/eagle.css explicitly.
• Themes are now in dist/ directory and the users should import them but things in src/ still also works.

[Zulko]

Thanks so much for this. For the record, I may not have the time to review this before long so I am blindly trusting you. I'll update the npm repo in a few days.