Warn on unused variables

[harrisont]

It would be a nice feature to warn on unused variables.

• Note that a variable should be considered unused if it is never read, even if it is written.
• Make sure to count function properties as read, even though it doesn't appear in the .bff file as a read.
• Follow code paths for all platforms, not just the current one (__WINDOWS__/__OSX__/__LINUX__).
• Add a setting to toggle the check, since some users will find it annoying, and it's backwards incompatible.

[harrisont]

Work in progress in branch issue-84.

Still remaining:

• Make sure to count function properties as read, even though it doesn't appear in the .bff file as a read.
• The setting currently defaults to enabled, which is a backwards incompatible change. Should it instead default to disabled?
• Add tests.

[harrisont]

Work in progress in branch issue-84.

Still remaining:

• Change from errors to warnings.
• Manual testing.
• Add automated tests.
• The setting currently defaults to enabled, which is a backwards incompatible change. Should it instead default to disabled?