Add ability to restrict modifications to an isolated portion of a yaml file

[alechenninger]

This will help interaction with YAML files that are partially human edited, and partially managed by monarch.

For example, the YAML might look like:

# human comments
human::managed::key: value

# --- Begin generated by monarch
monarch::managed::key: value
# --- End generated by monarch

# more human comments
another::human::managed::key: value

[alechenninger]

This is a similar goal but different solution to #15

The benefit here is that this can keep all non-monarch managed portions perfectly intact as long as the comments are not messed with and it is simpler to implement than writing a round trip parser/dumper in java or porting to python

[alechenninger]

Related: #44