* scalable commands column
* new multi-json loader
For both internal & external json loader
This commit also:
* applies -confdir to other formats, e.g. "yaml" in the future
* multiple assign of -confdir is accepted
* add flag to load confdir recursively
* config loader can have alias name
* json loader also accepts .jsonc
* add merge command
* add help topics for json merge, format loader
* format loaders don't panic
* apply lint style
* add merge test
* merge same tag in array, solve v2fly/discussion#97
* apply lint style
* merge code optimize
* fix merge cmdarg.Arg
* update cmd description
* improve merge logic
* fix zero value overwrite
* fix "null" lost after array merge
* code optimize
* fix merged slices not sorted
* code optimize
* add package doc
* fix a typo
