Tmux buffers for sending text

[t6d]

This pull request fixes two issues:

1. Sending arbitrary long texts is now possible (see also issue Can't paste more than around 30-lines into tmux buffer #44). Using tmux send-keys resulted in a tmux crash if too much data was pasted from vim to tmux as the command was simply too long to be interpreted.
2. Shell interpolation is prevented. Instead the text is pasted precisely as is. An issue I encountered before was that I was required to escape any sequence containing a $ sign before sending it using VimuxSendText.