Consolidate renaming mode to one option

[dbr]

Basically finish up PR #172 - instead of various boolean options like leave_symlink, always_rename etc, have one option like move_mode: "always_rename" or move_mode: "leave_symlink" etc

[dbr]

As per the linked PR, the available options would be:

# The method used to move files at configured destination
#
# * move (default)   - to move files
# * copy             - to copy files
# * hardlink         - to link files with a hard link
# * hardlink_or_copy - to hardlink files when possible otherwise, falls back to COPY
# * symlink          - to link files with a symbolic link
# * leave_symlink    - to move files and replace the old ones with a symbolic link to the new location