Implement automatic encoding detection for input.

Fixes #6. List of changes:

  * A new flag, `--utf8`.
  * A new function, `toStringGuess`.
  * `hGetContents`, `getContents`, `readFile`, and `handleIncludes`
    in LaTeX reader take an additional boolean argument specifying
    whether to guess encoding or not.
  * A new field in `WriterOptions` – `writerGuessEncoding` – presently
    used only by EPUB writer.
  * 2 new dependencies – charsetdetect-ae and text-locale-encoding.
  * Updated README.
Style.