Use of home (~) in reporter output file names doesn't work

[ecordell]

If you run with -r markdown -o ~/test.md, dredd exits with an error similar to:

error:  errno=34, code=ENOENT, path=/current-working-directory/Users/evan/test.md

[ecordell]

Looks like file.path.abspath(path) in the file package would fix this cleanly:

Expands ".", "..", "~" and non root paths to their full absolute path. Relative paths default to being children of the current working directory.