Add magazine document protobuf spec

[gezakerecsenyi]

The purpose of this branch is to solve #14 - i.e., create a definition format by which magazines can define their issue contents (similarly to/as an alternative to a .pdf file), as well as allowing for future issues to create a simple, straightforward implementation of such format.

To-do:

• Format
  • [x] Add page master support
  • [x] Add global mixin support
  • [x] Add page/section definitions
  • [x] Add simple, mobile-friendly styling options for images and text
  • [ ] Add front-cover definitions
• Documentation
  • [x] Create documentation README, describe basic intentions
  • [x] Document key terms/sections
  • [ ] Add implementation guide for magazine authors
  • [ ] Add implementation guide for app developers

[palkerecsenyi]

This is way more complicated than I was intending! For now, could you please just have simple definitions for images, text, and basic formatting, as described in the issue. No borders, positioning, custom text styling, etc. please, just a very slightly improved markup format. Thanks!