acme: Add package.

This adds a package wrapping the acme.sh script from
https://github.com/Neilpang/acme.sh in Uci config and hooks to interact
correctly with uhttpd.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

net/acme/files/acme.config

@@ -0,0 +1,11 @@
+config acme 'main'
+       option state_dir '/etc/acme'
+       option account_email 'email@example.org'
+       option debug 0
+
+config cert 'example'
+       option enabled 0
+       option use_staging 0
+       option keylength 2048
+       option update_uhttpd 1
+       list domains example.org