0.18.0

• Changed to only generating the .gleam output if the .matcha file has a newer modified timestamp than the .gleam
  file. This avoids constantly regenerating the output files which in turn avoids messing up the formatting if the
  output file had been formatted.

  Thank you to @perrygeo for noticing and fixing the issue.

• Switched from structopt to clap for command line options parsing. This should be transparent to the user.