v0.3.0

Bump version to 0.3.0

This change bumps the crate's version to 0.3.0. The following notable
changes have been made since 0.2.0:
- Added perfbuf-usage lint
- Added unrolled-for-loop lint
- Introduced public 'terminal' module with terminal related reporting
  functionality
- Introduced 'lint_custom_opts' function and 'LintOpts' type
  - Added support for specifying target kernel version
- Added 'terminal::Opts::color' attribute for colorized terminal output
- Fixed inconsistent indentation of context in multi-line match terminal
  reports
- Decreased size of logo used in README