Refactor location installation path code
Refactor location installation path code

Now it is possible to set the installation path by setting
POWERLEVEL9K_INSTALLATION_PATH. If set, this path will always be used.
If it was not set manually, we try to determine the path automatically.

andjscott/powerlevel9k,natemccurdy/powerlevel9k,MarvinTeichmann/powerlevel9k,shibumi/powerlevel9k,MarvinTeichmann/powerlevel9k,zer4tul/dotfiles,bhilburn/powerlevel9k,rjorgenson/powerlevel9k,zer4tul/dotfiles,dritter/powerlevel9k,rjorgenson/powerlevel9k,bhilburn/powerlevel9k,dritter/powerlevel9k,andjscott/powerlevel9k,shibumi/powerlevel9k,natemccurdy/powerlevel9k