see https://inkscape.org/en/doc/inkscape-man.html
**Note:** Running `inkscape` from the command line changes the working
directory to `#{staged_path}` (i.e.,
`/opt/homebrew-cask/Caskroom/inkscape/0.91-1/Inkscape.app`), which
means that all file arguments should be given as *absolute paths*.
There is a shim script in `Inkscape.app/Contents/Resources/script`,
but it doesn't solve this issue any further than simply running
`Inkscape.app/Contents/Resources/bin/inkscape`.
See also http://wiki.inkscape.org/wiki/index.php/MacOS_X#Inkscape_command_line
`md5`, `sha1`, `sha256` all take a hexdigest string, e.g:
sha1 'f645e9da45a621415a07a7492c45923b1a1fd4d4'
`no_checksum` takes no argument, and indicates there is no checksum
for this cask. This is *not recommended*, and should only be used for
casks that have no versioned downloads.
`brew cask install` will complain if there is no sum provided (unless
`no_checksum` has been invoked), or if the sums do not match. It will
provide the computed checksum so the cask can be easily amended.
Adapted from @passcod's work in 82cc199ae6
Victor Popkov