Bump version number to 0.5.0

Version 0.5.0 contains now:
 * #18 Add a "list" feature also with improved colouring #56
 * #39 sorted output for search
 * #17 configurable installation directory
 * #15 dryrun option does no longer download lib files
 * #52 store hashes for tags with
 * #46 executable name is now `impact` already since v0.3.0