Add limits to prevent abuse

[akissa]

What I did:

Investigating the package for use in a email scanning project, looked at the docs and the source.

What I expected:

Options to limit the max number of mime parts to be processed and max nested levels to descend.

What I got:

Cannot find any limits, meaning memory exhaustion could occur on a message with very many parts.

I suspect you could also run into issues with recursion limits being exceeded for deeply nested messages ?

Release or branch I am using:

master

[jhillyerd]

enmime doesn't have protection for that. Given that we hold parsed messages in memory, I think it's more likely we run out of memory with large attachments than run out of stack due to recursion.

Once #90 is implemented, we could have optional limits on parts, part size, and nesting depth.