Implement -v flag

[a8m]

I want to add support for the -v(--variables) operation that currently exists in the gettext version.
more info here

[ragboyjr]

@a8m so i guess this would also need to include the shell-format argument as well?

[icy]

Fyi, I found this tool is great, but it's more features than I need, so I wrote my own version https://github.com/icy/genvsub with some strict format ${VAR_NAME}, however the tool can support both options -v and shell-format by using -p regexp. Feel free to have a look. Many thanks :)