servo: Merge #4083 - Mach debugger flag (from mttr:mach_debugger_flag); r=mbrubeck

Borrows two python modules from mozilla-central to give mach the ability to detect and pass arguments to a system's preferred debugger, and adds `--debug` and `--debugger` flags to `./mach run`. This works almost like the functionality described [here](https://developer.mozilla.org/en-US/docs/Debugging_Mozilla_with_gdb), but at the moment it lacks a `--debugparams`  argument.

Links to borrowed files:
http://hg.mozilla.org/mozilla-central/file/c9cfa9b91dea/testing/mozbase/mozinfo/mozinfo/mozinfo.py
http://hg.mozilla.org/mozilla-central/file/c9cfa9b91dea/testing/mozbase/mozdebug/mozdebug/mozdebug.py

Source-Repo: https://github.com/servo/servo
Source-Revision: 989efd5a2820a152c5d5a2d42c9be3c30d79f469

servo/python/mach_bootstrap.py

@@ -11,6 +11,8 @@ import sys
 SEARCH_PATHS = [
     "python/mach",
     "python/toml",
+    "python/mozinfo",
+    "python/mozdebug",
 ]
 
 # Individual files providing mach commands.