Customizes tabline appearance: type, close button

This commit adds a couple of new settings so that it's possible to hide
the tab type (all the way to the right) and the symbol which represents
the close button.

The settings and their defaults:

    let g:airline#extensions#tabline#show_tab_type = 1
    let g:airline#extensions#tabline#close_symbol = 'X'

This pull request lets me customize the way the tab bar looks to pull off an appearance like this:

<img src="http://drop.emily.st/s/Screen%20Shot%202014-03-19%20at%2013.31.11.png" />