Support for line continuations

[vilya]

OBJ allows line continuations: if you put a backslash at the end of a line, parsing is supposed to continue on the next line as if it were part of the same line (Source: https://www.fileformat.info/format/wavefrontobj/egff.htm). fast_obj doesn't handle this properly and can seg fault when parsing files with line continuations.

It's pretty rare to encounter this in the wild and handling it properly would likely have a performance impact so maybe it's not actually worth supporting, but I thought it was at least worth mentioning.

Either way, thanks for making fast_obj available - it's a really nice little library!