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
his/her `~/.tmux.conf` file. For existing tmux users leveraging
powerline, this may prove to be a slight one-time nuisance immediately
after upgrading. 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.
Use scripts/powerline-config to source tmux configuration

Ref #872
Use POWERLINE_CONFIG_COMMAND environment variable
Update documentation

Closes #872
Merge pull request #880 from ZyX-I/powerline-config-script

[RFC] Powerline config script