v0.1.15

CLI: create function: support single-file functions (#474)

* CLI: create function: single-file functions

Make `--handler` unnecessary when creating single-file functions.
The base-image will try to guess the handler at function creation.
This is language dependent.

Function creation will fail for base-images not supporting this feature.

Also, update docs and base-image versions

* Bump base-image versions

* Error out when source is a dir and no handler is set