make it possible to intercept debug logs

[bjaglin]

Initial naive implementation of #74

[bjaglin]

Not very satisfied so far as:

• I went for a PrintStream signature which fits well with the "logging" use-case, but it's not consistent with the existing overload to intercept protoc streams.
• the overload cascades to all methods as everything is public static
• for any new functionnality, it would be very easy to log to sdtout by mistake even though the calling context asked for a specific stream

Maybe it's worth using a proper logging framework?

[os72]

This could fit with a bigger initiative to make the project more modular, yes less "public static". Pluggable logging, thread-safety, etc