change the imports of caddy server (#44)

Use go modules

Allow IPv6 file names to use = rather than : (#40)

Go doesn't allow colons in file names when module support is enabled.
This restriction is to acommodate systems like MS Windows where colons
in file names may have special meaning. So allow the use of equal-signs
in place of colons in file names that represent IPv6 addresses.

Resolves #39

Improves README, disallow more than one 'prefix_dir'. (#33)

I noticed that the `strict` directive wasn't documented at all. And
there wasn't really a good description of the syntax and other
directives.

This also modifies the code to disallow more than one `prefix_dir`
directive per block since more than one doesn't make sense.

This also modifies the code to allow more than one `country` directive
to match the behavior of `ip`. Thus you don't have to specify all the
country codes on a single line.

gofmt: minor changes

Fix #28