--colors and CARGO_TERM_COLORS support

[sourcefrog]

Fixes #178

• Add --color=always,never,auto
• Check that colors actually show up in our own CI, with CARGO_TERM_COLORS set
• Follow NO_COLOR and CLICOLOR_FORCE from https://bixense.com/clicolors/
• Also follow CARGO_TERM_COLOR
• Hook this into control of tracing color on stderr
• Hook this into styling of mutants and findings
• Test that listings, findings and trace output are styled with colors forced on
• Check auto looks at isatty: it does, by relying on the logic in the console crate
• Mention in docs
• ... including in the CI setup instructions & examples
• Test interactions of environment variables and options
• ANSI colors could be on or off independently on stderr and stdout: for example if stdout is redirected to a file then auto colors would be off, but we should still color stderr if it goes to a terminal
• Clear relevant env vars when running CLI tests so that they're deterministic
• Update wiki page, or maybe move it into comments or the design doc

See also https://github.com/sourcefrog/cargo-mutants/wiki/Handling-ANSI-colors