Allow custom exclusion of encodings, not just encodings to try first

[Leonard Richardson]

Feature request from https://groups.google.com/forum/#!topic/beautifulsoup/eHtTvyZ82jc:

I'm having an issue with guessing file encodings based on UnicodeDammit (w. chardet installed).

dammit.original_encoding tells me that the encoding is "maccyrillic" although it is utf-8.
Therefore, I'd like to exclude "maccyrillic" from the list of possible encodings to check for. The only option I've found is to explicitely define a list of suspicions (which doesn't work for my usecase).

[Leonard Richardson (leonardr)]

Feature added in revision 373.