Use *_log_options instead of access_log_format per #174.

access_log_options and error_log_options allow for custom log options
while retaining the existing log name scheme. They can be combined with
either a custom log name (access_log / error_log) or without custom log
names.
Add custom log_format to #174.

Allows for a log_format config line to be generated for use in either
the $access_log or $access_log_options parameters. Only one log_format
per vhost.