Supress tab numbers and improved uniq_tail formatter

Hi, in this pull request:

All changes are for tabline extension.

1. g:airline#extensions#tabline#show_tab_nr - allows to hide tab numbers in tab mode
2. convert slashes in buffer name from windows to unix style;
3. New formatter "uniq_tail_improved"
 - works in tab mode;
 - process only buffers with non unique file names;
 - shows only different parts of path, compact similar path tokens to '...', for example, if we have following four buffers opened:
<table>
  <tr>
    <th>buffer path</th><th>buffer displayed name</th>
  </tr>
  <tr>
	<td>zzz/vvv/fff/app.txt</td><td>zzz/.../fff/app.txt</td>
</tr>
<tr>
	<td>zzz/vvv/xxx/app.txt</td><td>.../xxx/app.txt</td>
</tr>
<tr>
	<td>ccc/vvv/fff/app.txt</td><td>ccc/.../fff/app.txt</td>
</tr>
<tr>
	<td>c:/sdsa/asd/asdsa/www/vvv/sss/app.txt</td><td>.../sss/app.txt</td>
</tr>
</table>

All new functionality tested in different use cases at windows and linux.