Text mode support (stage 2)

[Myriad-Dreamin]

Previous issue: https://github.com/mitex-rs/mitex/issues/35

• [ ] update README.md to clarify that for #130:

  mitex is well tested by fixtures extracted from oiwiki, which always use mimath (math mode), while you are using mitext (text mode), which is not well tested.

  Considering that, the introduction in README.md are misleading, since it wrongly tells mitex in text mode is also well tested. The introduction was correct as I wrote it when we only implemented math mode. However, once after introducing text mode, there should be quite misleading.

• [ ] handle several typical and commonly used environments such as document, figure, algorithm and description.
  • don't take too much design first, but consider them concretely.
• [ ] don't panic: randomly pick 10 papers from different research area, and convert them without panic.
• [ ] do stub: for those environments that are hard to implement, e.g. algorithm and tikz, we should parse them correctly and put warning boxes in place.
• [ ] beautify: correct visual effect, implement simple templates for them.
• [ ] 🎉