Add alias/symlink feature

Add a command to create a symlink for a secret. This effectively creates an alias for a given secret.

Symlinks should hold a relative path to a secret in the same password store.

Example:

# Create alias for site/google.com, named site/alias.google.com
prs ln site/google.com site/alias.google.com

# Move original file around (updates symlink)
prs mv site/google.com other/google.com

# Show contents of other/google.com through alias
prs show alias.google.com

Tasks:

• Make symlink files selectable/queryable
• Add ln command (#11 (closed))
• Add symlink support to remove command
  • Find symlinks to removed file, (ask to) delete as well (#12 (closed))
• Add symlink support to move command
  • When moving regular secret file, symlinks linking to it must be updated (#12 (closed))
  • When moving symlink secret file, relative path to target must be updated (#12 (closed))
• Add symlink support to duplicate command
• Add flag to list command to exclude symlinks (#13 (closed))
• Add housekeeping sub command to find (and delete) dead symlinks (#14)
• Add feature notice in README