Add platform option to enable mac use

[gamgi]

Add platform option to update.sh. This allows downloading the latest Chromium version for mac OS too.

Examples:

./update.sh mac
./update.sh linux
./update.sh           # defaults to linux

[ghost]

Hi! I recently forked this project for add more things, and your idea is really good. I hadn't thought about Mac OS X implementation until you said it. Thanks a lot for the idea, I already implemented in my fork where I use 'uname' to determine the operative system, without asking to the user what is your operative system. I put your user in the thanks of my readme.md. Thank you and greetings!