tmux: Enhance version checking for tmux

`tmux` configuration has become very fragmented between versions due to
a combination of new features and deprecation of older options. As such,
version checking for `tmux` became a requirement to allow maximum
functionality along with version-appropriate configuration. However,
wrapping nearly every line with `if-shell` is tedious and becomes
virtually unreadable.

This enhancement to `tmux` version-checking creates a logical
configuration file structure that reflects the development of `tmux`. As
additional configurations are added and `tmux` development continues,
this structure can be leveraged appropriately and extended as needed.

Rather than having multiple `if-shell` checks for the same version, each
version or version group with specific requirements can be checked only
once. This leads to simpler and fewer `if-shell` version checks. It also
reduces the ambiguity introduced by complex `if-shell` commands that
include an 'else' conditional.

A caveat to adding this enhancement is the additional requirement of the
end user to add a tmux variable definition (POWERLINE_BINDINGS_DIR) to
their `~/.tmux.conf` file. For existing `tmux` users leveraging
powerline, this may prove to be a slight nuisance. Without this
definition, there is no way to determine the location of the additional
`tmux` powerline config files that have been added with this
enhancement. The docs have also been updated in this commit to reflect
this initial configuration requirement change. Also added are some
additional troubleshooting notes that point to definition of this
variable as a potential issue. Powerline will continue to work without
the definition of POWERLINE_BINDINGS_DIR, but some of the functionality
will be reduced and the appearance incomplete (i.e. colors).

Ultimately, the caveat above is a small price to pay in exchange for a
framework that can be leveraged for proper version-appropriate
configuration of `tmux` going forward.