Separate Format Components

[threeal]

This pull request resolves #87 by introducing a format component that contains an errors_format target. The target itself contains an errors::format function and a fmt::formatter<errors::Error> implementation which is separated from the errors target.

This pull request also introduces a NOT_SUBPROJECT variable to allow the format component to know if the root project is not a subproject, allows the format component to build its test target.