Add `--symlinks` option

[overlookmotel]

This PR adds a new command line option --symlinks to follow symlinks when finding files in the directory being inspected.

Particularly useful in conjunction with the --as-asset-store option if you want to check if a VF DCP and OV DCP marry, and the two DCPs are stored in different locations / on different drives. You can create a new directory, symlink both the DCPs into it, and run dcp_inspect on that directory with the --symlinks option.