Major code rewrite, add support for download directory selection

Functions now return variables, which are then called in main and used as arguments for other functions. Globals have been removed. The user can now select their default download directory by typing the path when prompted.

The user will be prompted to create the directory ~/.git-chromium.
The latest snapshot downloaded has its revision number saved into ~/.git-chromium/LAST_INSTALL and the path to the download directory is saved into ~/.git-chromium/DOWNLOAD_DIR.