Add function #'ADD-LOCAL-PROJECTS-TO-MANIFEST

Fixes #83 

I tried to copy the same ideas of pathname handling and stream writing from other functions of the quicklisp-client package.

Maybe add to the docstring a tip saying to use `(ql:write-asdf-manifest-file)` and `(ql:register-local-projects)` before using `(ql:add-local-projects-to-manifest)` in order to keep all up-to-date?