Fixes #650 Correct window positioning defaults

[raven42]

Fixes #650
With default case having multiple vertically split windows, the tagbar
window should open on the far right of the screen. When the feature in
pull-request #630 was added, this behavior changed to open the tagbar
window relative to the current window. This commit will fix that by
allowing direct usage of the vim split options.

Validated the following cases for the old default values. These all
still behave the same way as prior to pull-request #630 when the tagbar
is activated in the active window shown in the below examples:

	" No options - tagbar on right, preview above
	+-----------------------------------+
	| preview                           |
	+--------+--------+--------+--------+
	| edit   | edit   | edit   | tagbar |
	|        | active |        |        |
	+--------+--------+--------+--------+

	" tagbar on left, preview above
	let g:tagbar_left = 1
	+-----------------------------------+
	| preview                           |
	+--------+--------+--------+--------+
	| tagbar | edit   | edit   | edit   |
	|        |        | active |        |
	+--------+--------+--------+--------+

	" tagbar on bottom, preview to right of tagbar
	let g:tagbar_vertical = 10
	+--------+---------------+--------+
	| edit   | edit          | edit   |
	|        | active        |        |
	|        +-----+---------+        |
	|        | tag | preview |        |
	+--------+-----+---------+--------+

	" tagbar on top, preview to the right of tagbar
	let g:tagbar_left = 1
	let g:tagbar_vertical = 10
	+--------+-----+---------+--------+
	| edit   | tag | preview | edit   |
	|        +-----+---------+        |
	|        | edit          |        |
	|        | active        |        |
	+--------+---------------+--------+

New values supported by #630 also all validated using the defined behavior

let g:tagbar_position = 'right'   " Behaves the same as no options
let g:tagbar_position = 'left'    " Behaves the same as g:tagbar_left = 1
let g:tagbar_position = 'bottom'  " Behaves the same as g:tagbar_vertical = <#>
let g:tagbar_position = 'top'     " Behaves the same as g:tagbar_left = 1 g:tagbar_vertical = <#>

[alerque]

Thanks @raven42, this is lookig good. I haven't actually had a chance to test drive it but the code looks sane.

@RoDuth have you tried this out yet in practice?

[RoDuth]

Thanks for your effort on this @raven42, works for me.

@alerque I've given it a quick run around the block (behaves as expected), and agree that the code looks good to me, happy for you to merge it.