rewrote major parts of `pwnlib.tubes.tube` for hugely increased performance.  introduced slight API changes in the process.
added argument `keepend(s)` to functions in `pwnlib.tubes.tube` recieving lines.
bugfix: `pwnlib.tubes.tube.recvline()` now uses the `timeout` argument when called with no `delims`.
removed per tube log levels from `pwnlib.tubes.tube` as the feature was hardly ever used and not very intuitive.  might reintroduce later.
`pwn` now also looks for environment variables starting with "PWNLIB_"; "PWNLIB_LOG_LEVEL" sets `context.log_level`. added a new flag "NOTERM".