feat: AutoGroup option

fix: hydrate pointer fields

It is a mystery how this has been a bug for so long.

fix: Change the application flags order avoiding panics

This commit solves issue alecthomas#264

fix: issue with symlinked temp dirs

Differentiate between omitted and empty default.

Fixes alecthomas#250.

fix: overly enthusiastic validation of tags

Do not add environment variable to help it is already present.

Fixes alecthomas#246.

Add test for alecthomas#244.

Add envprefix tag

Update docs.