Custom rows grouper

WIP - Make it easy to create an iterator of a text file with records of a custom, potentially multi-line format.

if a file is like:

    1234 record 1
        line 2
        line 3
     1234 record 2
        line 2
        line 3
        line 4
     1234 record 3

you could iterate over it and get a sequence of all the lines of each one:
first:

    1234 record 1
        line 2
        line 3

second:

     1234 record 2
        line 2
        line 3
        line 4