Release version 1.2

Changes since v1.1:

- #15: Use the portable C locale when parsing the PDF header.
- #10: Check if the input file exists before running `gs`.
- #7: Use the PDF version of the input for the output file.

Release version 1.1

Changes since v1.0:
- Add an optional argument to output a grayscale PDF.
- #3: Check for equal input and output files to avoid truncation.
- #4: Update README.md.
- #5: Replace optional positional arguments with option flags.

Release version 1.0

The shrinkpdf script has been available for a long time on its project
website. It has gone through a number of iterations, but was never
properly versioned.

The script is quite battle-tested by now, so let's create a first
official release.