Rename skipped_tabs_marker to overflow_marker

This is set to g:airline_symbols.ellipsis, which this commit also adds.

autoload/airline/extensions/tabline/tabs.vim

@@ -103,7 +103,7 @@ function! airline#extensions#tabline#tabs#get()
   let right_position = tabs_position + 1
   let remaining_space = &columns - s:strchars(s:evaluate_tabline(b.build()))
 
-  let skipped_tabs_marker = get(g:, 'airline#extensions#tabline#skipped_tabs_marker', '...')
+  let skipped_tabs_marker = get(g:, 'airline#extensions#tabline#overflow_marker', g:airline_symbols.ellipsis)
   let remaining_space -= 4 + 2 * s:strchars(s:evaluate_tabline(skipped_tabs_marker))
 
   " Add the current tab

autoload/airline/init.vim

@@ -70,7 +70,8 @@ function! airline#init#bootstrap()
           \ 'spell': 'SPELL',
           \ 'modified': '+',
           \ 'space': ' ',
-          \ 'keymap': 'Keymap:'
+          \ 'keymap': 'Keymap:',
+          \ 'ellipsis': '...'
           \  }, 'keep')
 
   if get(g:, 'airline_powerline_fonts', 0)

doc/airline.txt

@@ -710,7 +710,7 @@ with the middle mouse button to delete that buffer.
   let g:airline#extensions#tabline#tabs_label = 't'
 
 * change the symbol for skipped tabs/buffers (default '...') >
-  let g:airline#extensions#tabline#skipped_tabs_marker = '…'
+  let g:airline#extensions#tabline#overflow_marker = '…'
 
 * always show current tabpage/buffer first >
   let airline#extensions#tabline#current_first = 1