String preprocessor account regex catches only fully-capitalized/numeric account handles

[CogentMentat]

In tweetnlp/tweetnlp/util.py, the get_preprocessor function for unknown preprocessor type replaces user handles like so:

text = re.sub(r"@[A-Z,0-9]+", "@user", text)

but this catches only capitalized alphanumeric handles.

I have never submitted a pull request before, so if you want to let me try it out I can submit one - if of course you agree with the problem!