Add number prefix to directory names of categories/challenges to indicate order

When you start the IPython/Jupyter notebook at the top-level of the repo, as suggested by the readme, it presents the directory listing and it is not clear where to start and which directories contain challenges and which do not (e.g. the `templates` directory).

If you go into a challenge category directory, again, it is not clear in which (suggested) order the challenges (if any) should be tackled.

I suggest naming the category and challenge directories with a two-digit number prefix, so that they are ordered like in the readme. Example:

Categories:

```
01_arrays_strings
02_linked_lists
....
```

Challenges:

```
01_unique_chars
02_permutation
...
```