Implemented functionality for printing a flat representation instead of a tree.

`fsls "~/projects/*/README.md" --flat`

Will print

    /home/akoenig/project1/README.md
    /home/akoenig/project2/README.md
    /home/akoenig/project3/README.md
    /home/akoenig/project4/README.md

Hope you like it :)