Make 'sh:' directives a bit more usable

[doronbehar]

To fix #16 , and to make sh: work for (I hope) most edge cases, some api changes are needed - above all: use a different placeholder for the file name in command definitions. In this PR %pistol-filename% is used instead of %s.

TODO:

• Consider implementing a somewhat backwards compatible interface
• Release a new version if this will be a breaking change.
• If releasing, use the GitHub release notes feature.
• If releasing, put a banner in the README to indicate that for users with broken configs.
• Update README in general.

Other then that, the version presented in this PR (essentially what's in the sh-fixes-and-new-api branch) should be usable, after switching the configs from using %s to %pistol-filename%.