Add a way to include development dependencies

[abergeron]

I've added an option to include the development dependencies in the generated environment file.

This would be really useful for me since we want to switch a project I develop on to poetry but I would prefer to keep using conda for it.

[dojeda]

Hi @abergeron, Thank you for this contribution! It is a simple, yet useful feature and the unit test is highly appreciated.

I was not planning on releasing a minor version update soon but if you need/want a simple version reference on your project definitions, let me know and I will release an update. If you have thought about additional contributions, I can hold (I know I have one feature request and one minor bug that I found recently).

[abergeron]

I plan to do something else to handle extras also. If you want to do a release after that it would be nice.