tmux: Shell variable definition for tmux version

Add variables for tmux major and minor version numbers. These can
subsequently be used to test for tmux version compatibility.
tmux: Check version for prefix active colorization

Prefix active colorization feature introduced by pull request #863
causes undesired behavior on older versions of tmux. Add version
checking and revert to old powerline configuration if the tmux version
is less than v1.8. tmux v1.8 was the first version to support the
client_prefix format variable.
tmux: Check version for last window indication

Last active window indication feature introduced by pull request #864
causes undesired behavior on older versions of tmux. Add version
checking to determine which method for feature implementation to use,
dropping the feature entirely for tmux versions less that v1.8. For tmux
v1.9, use the newer 'window-status-last-style' option rather than the
deprecated (starting with v1.9) 'window-status-last-fg' option. Ideally,
the other `foo-{attr,fg,bg}` options used in the `tmux` powerline config
file should also be version checked and replaced with the corresponding
`foo-style` option as well.
Merge pull request #870 from austinbeam/feature/869-tmux-version-checking

tmux version checking (Issue #869)