JUnitFormatter marks skipped tests as failures in strict mode

Let the option strict make JUnitFormatter to mark skipped tests as
failures. Use a new interface StrictAware to determine which formatters
the strict option should be passed to, following the example of the
interface ColorAware determining which formatters the monochrome option
is sent to.
Merge pull request #543 from brasmusson/junit-formatter-strict

Let the JUnitFormatter mark skipped tests as failures in strict mode