Add support for the user to specify custom headers to be indexed (and
re-index these for existing messages at the next database upgrade).
-Build system
-------------
-Fix to allow a non-source-directory build. For example, the below
-should be made to work:
-
- mkdir build_dir
- cd build_dir
- ../configure
- make
-
Test suite
----------
Achieve 100% test coverage with the test suite.