* add func to set level for specific logger
* add endpoints to modify log level
* initialize base logger with IndependentLevels
* test to ensure other loggers remain unchanged
* add DELETE loggers endpoints to revert back to config
* add API docs page
* add changelog entry
* remove extraneous line
* add log level field to Core struct
* add godoc for getLogLevel
* add some loggers to c.allLoggers
* Read config before creating logger when booting vault server
* Allow for specifying log output in JSON format in a config file, via a 'log_level' flag
* Create parser for log format flag
* Allow for specifying log format in a config file, via a 'log_format' flag. Also, get rid of 'log_json' flag.
* Add 'log-format' command line flag
* Update documentation to include description of log_format setting
* Tweak comment for VAULT_LOG_FORMAT environment variable
* add test for ParseEnvLogFormat()
* clarify how log format is set
* fix typos in documentation
