Extract Additional Warnings and Errors about Estimation

Hi Michael and others -

I have here a first go at extracting the warnings and errors in the Mplus output file that typically come after the SUMMARY OF ANALYSIS. These errors are typically about how the estimation went. The motivation is that seeing these errors is really important to me typically, and I don't want to gloss over them in an automated analysis. My goal is to just provide the code here and start a discussion - you don't necessarily have to use it if you don't want to. 

My solution works by capturing the messages before and after SUMMARY OF ANALYSIS by searching for any instances of the word WARNING, or any other estimation error flags which sometimes occur if the model estimation didn't terminate normally. I try to make it a little more general by then capturing text between the last error flag, and one of several "ending flags" (typically either MODEL FIT or MODEL RESULTS). The same warnerr object is built in a similar way as before.

Commit 4: I've re-factored to make the code more clear and extendable, and so that it works for a wider variety of cases.