Do not use argparge for parsing environment

This PR changes protocol used to connect between powerline clients and powerline daemon to make powerline daemon work faster. I have only relative numbers from profiler though (2/3 of rendering process wasted for parsing socket input were replaced with 1/6), but this part has to be changed as early as possible because it touches used protocol.

Reason why I was looking at profiling at all: shell integration tests make tmux client they run in blink which should be caused by either constant `refresh -S`, powerline-daemon busy by processing shell tests or both. This change did not fix anything regarding the issue though, but I at least removed the most obvious performance problem (I knew the culprint before running the profiler, yet did not expect it to take 2/3 of time).