fixed typo
Removed '=-' from short options

Usually short options do not have the form `docker tag -t=image`. So I
changed it to `docker tag -t image`.
A workaround for BSD sed/awk on OSX

This looks for gawk and gsed. If they exist then use them
in preference to awk and sed.

On OSX, you would want to do: `brew install gnu-sed gawk`
Merge pull request #22 from docwhat/osx-workaround

A workaround for BSD sed/awk on OSX
Simplify wrappers around awk/sed.