Add configuration option for an additional search path for .lst files

[cschlote]

USECASE:

Right now the Coverage files are expected to be located in the root directory of my dub project.
When the build system aranges things so that the .lst file are located in e.g. ./build/coverage then code-d won't apply coverage to the 'background'.

Wouldn't it be nice, if .lst coverage data could be stored somewhere else then in project top dir?

PROPOSAL

Add configurable extra paths where .lst files are stored.