Change Treeprocessor to TreeProcessor and create an alias for backwar…

[venthur]

…ds compatibility.

Closes: #815

[venthur]

Not sure which of the other classes matching *TreeProcessor need an alias too, just let me know, and I'll update accordingly.

[waylan]

I'm inclined to not merge this as the current capitalization matches the other classes (preprocessor, postprocessor, etc.). Anyone else have any thoughts?

[facelessuser]

FYI, BlockProcessor is an outlier: https://github.com/Python-Markdown/markdown/blob/master/markdown/blockprocessors.py#L59. I'm not sure which way you'd like to normalize class names 🤷🏻.

[waylan]

So it would appear that for the least amount of change we would change BlockProcessor => Blockprocessor, which would result in a consist style.

If we went the other way, we would need to change everything except BlockProcessor. Although, I can see an argument for not camel casing Preprocessor as that would be a single work in normal English prose. I suspect that is where the current style of not camel casing any of them case from. BlockProcessor was certainly a later addition and perhaps we forgot about that earlier style choice.

[venthur]

I'm fine either way, I've now also created #1290 that changes spelling for the BlockProcessor to Blockprocessor