List compile size, warning count, and compilation success deltas in the report

[per1234]

The change in program storage space, dynamic memory usage by global variables, warning count, and compilation success since the last commit should be shown in the report.

I think the best way to accomplish this is to do each verification twice, once using the code at the tip of the branch, then again after checking out the previous commit. The drawback of this is it will double the compilation component of the build duration, which is already problematic when a lot of tests are necessary.

The feature should be optional.

I need to make it work for comparisons between commits in the following usages:

• libraries (installed via install_library with no arguments)
• sketches (compiled from $TRAVIS_BUILD_DIR)
• hardware packages (installed via install_package with no arguments)