Add 'honor_cipher_order' option in ssl part

This option can be used to set to true/false the 'honor_cipher_order' parameter
on the ssl socket. This option was introduced in SSL application in Erlang/OTP
17.0, so it will be ignored by Yaws for previous releases. When supported, this
option is set by default to true.
Fix features detection when rebar is used
Support subconfig/subconfigdir directives in the server part
Add --configtest option in Yaws script to check a configuration file
Fix yaws_debug module to get debug dump on recent Erlang/OTP releases
Update known dialyzer warnings