Make it possible to enable the debug log prior to initialization

[gadenbuie]

It's currently possible to enable message debugging, but only after the chromote session is initialized.

b <- ChromoteSession$new() 
b$parent$debug_messages(TRUE) 

In cases like #148, where presumably Chrome is successfully launched but fails to run some of the setup commands, it may not be possible to enable debug logging early enough to print meaningful output.