Add support for Platform plugins to have settings.

Add two initial settings for the PlatformDarwinKernel plugin,

plugin.platform.darwin-kernel.search-locally-for-kexts [true|false]
plugin.platform.darwin-kernel.kext-directories [directory list]



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178846 91177308-0d34-0410-b5e6-96231b3b80d8