Naming conventions

The interface is named according to some of the predefined specifications, which is more convenient to understand and easy to use.

It's according to the following rules:

Interfaces	Description
is_/has_ + xxx	Condition interfaces
set_ + xxx	Set and override the previous settings
add_ + xxx	Set and append settings
…s + xxx (plural)	Support multi-parameters, .e.g：add_files("*.c", "test.cpp")
on_ + xxx	Set and override builtin script
before_ + xxx	Set and run this script before running builtin-script
after_ + xxx	Set and run this script after running builtin-script
scope("name")	Define a description scope, .e.g target("xxx"), option("xxx")
scope/settings	Indentation with spaces