Read template parameters from environment variables.

[Globegitter]

Here is a quick PR to fix https://github.com/InQuicker/ktmpl/issues/16. This is my first rust code so not sure how ideal it is. If you are happy with this code/future I am happy to ammend the README and add a test.

It might also be nice to add the base64 feature somehow but this is a simple minimum implementation I will now use myself.