Add option to preserve LookML file comments in `lkml.load`

[yaelmi3]

Some workflows need access to the comments at the top of a LookML file (e.g., autogenerated notes or metadata). Currently, lkml.load() discards this preamble.

Proposal:
Add an optional flag (e.g. keep_prefix) so the returned dict can include the raw comment/whitespace preamble:

lkml.load(raw, keep_prefix=True)
# => {"views": [...], "_prefix": "# This file is generated automatically\n..."}

suggested implementation can be found in this PR :)