Improve installation performance using multiprocessing

Added

• Add v5.12.2, v6.0.0 as known versions.(#176, #177)
• Support --archives option on src installation.

Changed

• Use multiprocessing.Pool instead of concurrent.futures(#178)
• Refactoring whole modules. (#179)
• Split old changelogs to CHNAGELOG_prerelease.rst
• Drop an upper limitaion (<0.11) for py7zr.(#183)

Fixed

• When we used "-m all" to download doc or examples, Qt sources are also downloaded(@Gamso)(#182)