Add ability to report compilation warnings, optionally fail on them

[ianfixes]

verify_sketch stdout:
Invalid library found in /Users/ikatz/Documents/Arduino/libraries/arduino_ci: no headers files (.h) found in /Users/ikatz/Documents/Arduino/libraries/arduino_ci
Invalid library found in /Users/ikatz/Documents/Arduino/libraries/CompileSomething: no headers files (.h) found in /Users/ikatz/Documents/Arduino/libraries/CompileSomething
Invalid library found in /Users/ikatz/Documents/Arduino/libraries/arduino_ci: no headers files (.h) found in /Users/ikatz/Documents/Arduino/libraries/arduino_ci
Invalid library found in /Users/ikatz/Documents/Arduino/libraries/CompileSomething: no headers files (.h) found in /Users/ikatz/Documents/Arduino/libraries/CompileSomething
Sketch uses 22084 bytes (4%) of program storage space. Maximum is 524288 bytes.

This can be found in the ret[:err] of ArduinoCmd.verify_sketch:

Loading configuration...
Initializing packages...
Preparing boards...
Verifying...
/path/to/arduino_ci/spec/FakeSketch/FakeSketch.ino: In function 'void setup()':
/path/to/arduino_ci/spec/FakeSketch/FakeSketch.ino:1:21: warning: statement has no effect [-Wunused-value]
 void setup(void) { 0; }
                     ^
/path/to/arduino_ci/spec/FakeSketch/FakeSketch.ino: In function 'void loop()':
/path/to/arduino_ci/spec/FakeSketch/FakeSketch.ino:2:20: warning: statement has no effect [-Wunused-value]
 void loop(void) { 0; }

Would need to

1. get a regex that can find warning lines reliably
2. need to reject lines that aren't part of the current project
3. decide how to store them
4. decide how to configure errors against them

[ianfixes]

Issue #100 🎉