feat: Add length option for shorten_path (#886)

* feat: add `shorten_len` option for path shortening

- adds option to configure the length of shortened parts of filenames
- only affects paths when "shorten" is in `path_display`

* chore: revert rebase for `path.lua` to 876bed9

* refactor: replace `shorten_len` with the `shorten` key in `path_display`

- also deprecates `utils.path_shorten` and passes straight to `plenary`s `Path:shorten`

* feat: allow `path_display` to handle table keys, as well as strings

doc/telescope.txt

@@ -179,6 +179,15 @@ telescope.setup({opts})                                    *telescope.setup()*
         - "shorten"   only display the first character of each directory in
                       the path
 
+        You can also specify the number of characters of each directory name
+        to keep by setting `path_display.shorten = num`.
+          e.g. for a path like
+            `alpha/beta/gamma/delta.txt`
+          setting `path_display.shorten = 1` will give a path like:
+            `a/b/g/delta.txt`
+          Similarly, `path_display.shorten = 2` will give a path like:
+            `al/be/ga/delta.txt`
+
         path_display can also be set to 'hidden' string to hide file names
 
         path_display can also be set to a function for custom formatting of