Use a FileTree to walk and validate all dotfiles

[RaphGL]

This is a custom data structure written specifically to solve issue #31

• Add and remove items
• Retrieve all nodes of a groups
• Remove all nodes of a group
• Add doc comments on how the functions should be used
• Allow combinators to be used to apply functions to either symlinked or not symlinked groups all in one iteration, instead of having to walk the data structure over and over again
• Canonicalize paths (remove unnecessary nodes, mark parent node as not symlinked if at least one child is not symlinked, etc)
• Consider reusing items marked as None in tree instead of allocating more memory
• Validate group names with dotfiles::is_valid_groupname
• Refactor symlinks.rs to use the new data structure