Attmap

Fixed
- Tweaked map type conversion upon value insertion

Added
- `EchoAttMap` as alias for `AttMapEcho`; see [Issue 38](https://github.com/pepkit/attmap/issues/38)
Fixed
- In any `OrdAttMap, for membership (`__contains__`) consider items added via attribute syntax.
- Prevent duplicate key/attr iteration in any `OrdAttMap`.
- Allow item and attribute syntax to equivalently mutate a map; see [Issue 50](https://github.com/pepkit/attmap/issues/50)
- Type conversion and merger of inserted mappings

Added
- Export base `AttMapLike`.
Changed
- By default, add trailing newline to YAML rendition of an attmap instance; [Issue 48](https://github.com/pepkit/attmap/issues/48)
- Better API docs
Fixed
- Do not replace double slash in URL with single slash. See [Issue 46](https://github.com/pepkit/attmap/issues/46)

Added
- `get_yaml_lines` to get collection of YAML-ready lines from any attmap
- `to_dict` to convert any attmap (and nested maps) to base `dict`
- `to_yaml` to represent any attmap as raw YAML text
Changed
- `PathExAttMap` defers expansion behavior to retrieval time.

Fixed
- `OrdAttMap` and descendants now have data updated via `__setattr__` syntax.

Changed
- `OrdPathAttExAttMap` is now `PathExAttMap`.