Detect and use base-index and pane-base-index values

[ethagnawl]

This feature makes rmuxinator behave as expected when the user's base-index and/or pane-base-index values are values other than 0 (tmux's default).

Closes #21 and #25.

Makes obsolete #32 (kudos to @kjkeller, as I borrowed from their solution and it was very helpful in bringing me back up to speed).

[ethagnawl]

• unify get_*_index methods
• test/handle edge cases
• write test(s)
• normalize pane/base-index variable names
• decide whether or not these values should live within config or similar meta config struct
• fix window-name off-by-one bug (See below)

If the tmux server is configured to use a base-index greater than 0, the first window is named incorrectly.

Given the following config file:

name = "debug"
[[windows]]
  name = "one"
[[windows]]
  name = "two"

When base-index 0:

cargo run debug Foo.toml 
...
debug _window_name: "one"
tmux new-session -d -s debug -n one
tmux new-window -t debug:1 -n two
tmux -u attach-session -t debug

When base-index 1:

cargo run debug Foo.toml 
...
debug _window_name: "two"
tmux new-session -d -s debug -n two
tmux new-window -t debug:2 -n two
tmux -u attach-session -t debug

[ethagnawl]

This will be included in an upcoming release -- probably 2.0.0.