check a plugin is valid in use().
rename captialize to dash2camel.
add test to check a plugin must have isactive function.
fix ensured-plugig always raised exception.
let initialize functoin is not neccessary to a plugin.
add test of plugin cli option processing hooking.
add test of plugin initialize.
Merge branch 'feature/plugin-mechanism-experimental' of github.com:clkao/pgrest into feature/plugin-mechanism-experimental