Add more content to "Working w/ text"

- No idea why Index changed here; I didn't make any explicit edits to it.
  Cleary the JSON format of IPython Notebooks is more malleable than I thought!
Refactor: New Index; move old Index to Preparations
Prepare for roundtripping via Markdown

- Create md version of Working with Text (aleady slightly edited and fixed).
- Create first version of roundtripping scripts, with a short README.
Roundtrip Working with Text via Markdown
Fixup: Properly split Working w/ Text/Strings

- Somehow, the "text" half of the old "Working with Text" went missing when
  I converted it. Now reintroduced.
- There's now a Working with Text and a Working with Strings. Both has .ipynb
  and .md versions. All fixed, I hope.
Make scripts work with wildcarded arguments

- Process multiple .ipynb or .md files sequentially in one script invoation.
  Had to mess with $IFS since filenames may have spaces.