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.

Allow `-` for stdout in path mapper (alecthomas#147)

More group formatting tweaks.

Add a bit of extra spacing if group description is provided.

See alecthomas#135.

Support a simplified `kong.Groups{<key>: <title>}` way of specifiying…

… groups.

See alecthomas#135.

Ensure values aren't nil before decoding.