Prevent empty links caused by link-directive

This addresses the issue where a `{{link('/')}}` will get expanded
into an anchor tag with no inner content by forcing the leading `/`
in the expansion.

Fixes #35

cmgr/loader.go

@@ -224,7 +224,7 @@ func (m *Manager) validateMetadata(md *ChallengeMetadata) error {
 		r = `<a href='{{url("${1}")}}' download>${2}</a>`
 		s = urlForRe.ReplaceAllString(s, r)
 
-		r = `<a href='{{http_base("${1}")}}:{{port("${1}")}}/${2}' target='_blank'>${2}</a>`
+		r = `<a href='{{http_base("${1}")}}:{{port("${1}")}}/${2}' target='_blank'>/${2}</a>`
 		s = linkRe.ReplaceAllString(s, r)
 
 		r = `<a href='{{http_base("${1}")}}:{{port("${1}")}}/${2}' target='_blank'>${3}</a>`