Making directives externally registerable

templates: Add .Markdown directive

This allows any template to use:
{{.Markdown "filename"}} which will convert the markdown contents
of filename to HTML and then include the HTML in the template.

markdown: enable definition lists

When the requested host is not found, log the remote host.

letsencrypt: properly retrieve hostname from request.

fastcgi: IPv6 when parsing r.RemoteAddr

basicauth: fixed 'go vet' printing function value

More tests are needed for the other environmental variables.
These tests were specifically made for testing of IP addresses.

fastcgi: Parse address from fastcgi directive, pass results to Dial()

This allows scheme prefixes "tcp://" and "fastcgi://" in configuration.

Fixes #540

wrap lines to 80

also update copyright year.

Challenge names now have their own type and constants

proxy: add a insecure_skip_verify option - closes #320

letsencrypt: Fix perm of user key

gzip: Add .svg to default ext list