Fix Pry.config.ansicon

Fix 3 topics
- delete command line option `no-ansicon` to .pryrc only
- change config option name from `ansicon` to `load_win32console`
- update warning message in Pry#start
Make Pry.load_win32console to private.
Create Pry::Platform and remove some methods.

Remove these methods from Pry::Helpers::BaseHelpers to Pry::Platform
- windows?
- windows_ansi?
- mri?
- mri_20?

Other methods will remove to Pry::Platform later, in this commit remove
methods which are needed to only Pry.load_win32console.