Provide control over log level

[keith]

Some log messages like this:

>>> //external links to the wrong place. Automatically deleting and relinking...
>>> Automatically added //external workspace link:
    This link makes it easy for you--and for build tooling--to see the external dependencies you bring in. It also makes your source tree have the same directory structure as the build sandbox.
    It's a win/win: It's easier for you to browse the code you use, and it eliminates whole categories of edge cases for build tooling.

Seem very useful for folks who are just starting to use this tool. But for a large team like ours where we require the use of the external symlink, this log is just noise that distracts folks from real potential issues. I think it would be nice to be able to tune the log level for eliminating some of these