issue-90 - to_string_converter in write_conf output

@twobraids r?

The issue https://github.com/mozilla/configman/issues/90 explains it well. Hopefully. 

Note, that now, when you run `--admin.print_conf=ini` and one of the options has a specified `to_string_converter` it is included in the commented-out output. E.g.:

```
# name: a
# doc: the doc string
# converter: configman.tests.test_val_for_configobj.dict_decoder
# tostring: configman.tests.test_val_for_configobj.dict_encoder
# Inspect the automatically written value below to make sure it is valid
#   as a Python object for its intended converter function.
a='one:One' 
```

I chose the word "tostring" which I don't love but can't think of a better one.