`stdin` mode

[akaihola]

Fixes #239.

• [x] Design decision: --stdin-filename vs. ~~unique file in the src argument with an --stdin option~~ (see discussion in #239)
  • [x] Add --stdin-filename command line argument, or
  • [x] ~~Ensure unique file in src~~
• [ ] Ensure rev2 == ":WORKTREE:"
• [ ] Ensure no positional file arguments
• [ ] Read stdin instead of file
• [ ] Update --help in README.rst
• [ ] Change log
• [ ] Tests