Supporting other platforms than Macosx

[jeroenvandijk]

Since the tool is developed on a Mac we can guarantee it works.

For other platforms, provide the following

• Fail with an error message if we don't know and suggest to create an issue
• Provide an alternative way to opening the browser

E.g. the following on Linux would fail with an error message

aws.console --incognito --browser brave
Don't know how to open brower 'brave' in incognito mode. Please create an issue if you have a suggestion 

https://github.com/jeroenvandijk/aws-console/new

In the meantinme you can add a custom command like 

aws.console --open-browser [a b c]