Set submodules to specific tags where possible.

alternate: 2.18-doc
bufexplorer: 7.3.5
csapprox: 4.00
figutive: v1.2
gundo: v2.4.0
matchtag: HEAD
nerdtree: 4.2.0
omnicppcomplete: 0.41
pathogen: v2.2
powerline: HEAD
solarized: HEAD
tagbar: v2.5
wombat: 2.0