Better configuration options

[docwhat]

The current auto discovery mechanism isn't ideal.

• custom_copy_command is currently a "fallback" copy command. A lot of people want an override instead. Ideally a user should be recommended a choice, but it shouldn't be dictated. This would also solve the "I need a slightly different copy command" or "my version is older/newer" problems. See #87 for more info.
• The helpers.sh code for managing configuration options is cumbersome and spread around (i.e. there are tmux names and defaults at the top and functions for each variable. It would be better to have something similar to a DSL.
• I'd prefer if all the options had a common prefix to prevent collisions and to make.tmux.conf easier to read.