Use Jekyll _data folder to generate pages

[simonpai]

Using Jekyll's data function data building can be further simplified.

• All normal data should be written into _data folder.
• All page content should be generated from {{ site.data.* }}.

[simonpai]

• gulpfile.js
  • write to _data/pesticide/index.json
  • write to _data/pesticide/entries/[id].json
  • write to _data/pesticide/usages.json
  • write to _data/pesticide/formulations.json
  • expose them to data folder in another task (?)
• view
  • _layout/pesticide-index.html -> pesticide/index.html
  • _layout/pesticide-formulation.html -> pesticide/formulations.html
  • pesticide/[id].html source front matter prints only id.