This creates a new kind of text splitter for markdown files.
The user can supply a set of headers that they want to split the file
on.
We define a new text splitter class, `MarkdownHeaderTextSplitter`, that
does a few things:
(1) For each line, it determines the associated set of user-specified
headers
(2) It groups lines with common headers into splits
See notebook for example usage and test cases.