xmake v2.1.5 released, lots of new feature updates

This release introduces a number of new feature updates, as detailed in Some new features of xmake v2.1.5.

If you want to known more usage, please see online documents。

Source code: Github, Gitee.

#83: Add add_csnippet and add_cxxsnippet into option for detecting some compiler features.
#83: Add user extension modules to detect program, libraries and files.
Add find_program, find_file, find_library, find_tool and find_package module interfaces.
Add net.* and devel.* extension modules
Add val() api to get the value of builtin-variable, .e.g val("host"), val("env PATH"), val("shell echo hello") and val("reg HKEY_LOCAL_MACHINE\\XX;Value")
Support to compile the microsoft resource file (.rc)
Add has_flags, features and has_features for detect module interfaces.
Add option.on_check, option.after_check and option.before_check api
Add target.on_load api
#132: Add add_frameworkdirs api
Add lib.detect.has_xxx and lib.detect.find_xxx apis.
Add add_moduledirs api
Add includes api instead of add_subdirs and add_subfiles
#133: Improve the project plugin to generate compile_commands.json by run xmake project -k compile_commands
Add set_pcheader and set_pcxxheader to support the precompiled header, support gcc, clang, msvc
Add xmake f -p cross platform and support the custom platform

#87: Add includes and links from target deps automatically
Improve import to load user extension and global modules
#93: Improve xmake lua to run a single line command
Improve to print gcc error and warning info
Improve print interface to dump table
#111: Add --root common option to allow run xmake command as root
#113: Privilege manage when running as root, store the root privilege and degrade.
Improve xxx_script in xmake.lua to support pattern match, .e.g on_build("iphoneos|arm*", function (target) end)
improve builtin-variables to support to get the value envirnoment and registry
Improve to detect vstudio sdk and cross toolchains envirnoment
#71: Improve to detect compiler and linker from env vars
Improve the option detection (cache and multi-jobs) and increase 70% speed
#129: Check link deps and cache the target file
Support *.asm source files for vs201x project plugin
Mark add_bindings and add_rbindings as deprecated
Optimize xmake rebuild speed on windows
Move core.project.task to core.base.task
Move echo and app2ipa plugins to xmake-plugins repo.
Add new api set_config_header("config.h", {prefix = ""}) instead of set_config_h and set_config_h_prefix

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