Automatic filter selection based on extension.

Hi,

Aspell now offers modes but their usage is bit clumsy. One of possible
improvements would be do automatic filter selection based on filename. 
It could be enabled for example by --mode=auto or something like that.

I noticed second possible improvement that in ccpp mode we get lot of
false positives due of variable names etc. If we first add word from
rest of code into temporary dictionary then we can easily check
references to pieces of code.