Add support for reading the .txt format useful for authoring crosswords.

[viresh-ratnakar]

The .txt format (http://www.litsoft.com/across/docs/AcrossTextFormat.pdf) offers a convenient
way to author a crossword puzzle using any text editor. With this commit, we can author a .txt
file and convert it to the .puz format easily, like this:
import puz
p = puz.read('puzzle.txt')
p.save('puzzle.puz')

[louispotok]

@viresh-ratnakar thank you for implementing this! @alexdej any objectio n to merging it?

If anyone else is interested, it looks like there is also a new "Across Puzzle V2" format. I used this commit on a file in that format, and it worked after the following changes:

• the <ACROSS PUZZLE> tag is now `'
• there is a new "rebus" section between GRID and ACROSS, I deleted it in the puzzle I was converting since it was unused, but I don't know how I would handle a non-empty one.
• tobytes failed because the first element in self.data was an empty string (rather than a bytes object). I just changed the call to return b''.join(self.data[1:])