Explore using clap

[AljoschaMeyer]

This is what clap-based modules would look like. How do you feel about giving these more verbose usage instructions in the list of modules?

I will refactor everything to use clap before merging this after we settle on a style (for the readme in particular). Right now I've taken the output of extremely --help, removed the windows-specific bits, and removed the --help option.

Compilation time hasn't been noticeably affected by deriving Parser for just a single module.

For reference, here is the output of extremely --help on my machine:

$ ../target/debug/extremely --help
Linearly interpolate each sample toward +/-1

Usage: extremely.exe [T]

Arguments:
  [T]  How strongly to interpolate samples toward +/- 1 [default: 1]

Options:
  -h, --help  Print help information

[AljoschaMeyer]

Reverted (commented actually) the changes to the readme; this now simply adds clap as a dependency and changes extremely to use it.

Merging so that further pull requests can use clap.