Stop treating tmux special.

It's not been necessary to use the tmux-escaping for a while, and causes
problems like we see in #40.  Lets flip things around.  If your tmux
needs it, then enable it with `g:togglecursor_enable_tmux_escaping`.

doc/togglecursor.txt

@@ -68,21 +68,11 @@ g:togglecursor_force    Force togglecursor to use a particular mechanism to
                         uses the DESCCUSR escape sequence) and 'cursorshape'
                         (what Konsole uses).
 
-                                                 *togglecursor_disable_tmux*
+                                             *togglecursor_enable_tmux_escaping*
 
-In some versions of tmux, the passthrough handling appears to be slightly
-broken.  When you are on the last line of a short file, and delete a line with
-'dd', tmux appears to insert an extra line into the mix and distorts the
-terminal.  It appears to be fixed in tmux 1.7.  If you'd like to disable using
-tmux passthrough support to try and change the cursor, do the following in
-your vimrc: >
-
-    let g:togglecursor_disable_tmux = 1
-
-The default value for |togglecursor_disable_tmux| is 0.
-
-Note: options should be overridden in your vimrc.  Changing them after Vim has
-loaded will have little or no effect.
+tmux has supported the ability to directly change the cursor for some time now.
+If you're using an old version that cannot handle this, then you can enable the
+old escaping behavior.  This shouldn't be necessary with modern tmux.
 
                                                  *togglecursor_disable_neovim*
 Note: This option is no longer supported.  Neovim adopted a different mechanism

plugin/togglecursor.vim

@@ -40,8 +40,6 @@ let s:xterm_blinking_block = "\<Esc>[0 q"
 let s:xterm_blinking_line = "\<Esc>[5 q"
 let s:xterm_blinking_underline = "\<Esc>[3 q"
 
-let s:in_tmux = exists("$TMUX")
-
 " Detect whether this version of vim supports changing the replace cursor
 " natively.
 let s:sr_supported = exists("+t_SR")
@@ -119,10 +117,17 @@ if !exists("g:togglecursor_leave")
     endif
 endif
 
-if !exists("g:togglecursor_disable_tmux")
-    let g:togglecursor_disable_tmux = 0
+if !exists("g:togglecursor_enable_tmux_escaping")
+    let g:togglecursor_enable_tmux_escaping = 0
+endif
+
+if g:togglecursor_enable_tmux_escaping
+    let s:in_tmux = exists("$TMUX")
+else
+    let s:in_tmux = 0
 endif
 
+
 " -------------------------------------------------------------
 " Functions
 " -------------------------------------------------------------
@@ -134,7 +139,7 @@ function! s:TmuxEscape(line)
 endfunction
 
 function! s:SupportedTerminal()
-    if s:supported_terminal == '' || (s:in_tmux && g:togglecursor_disable_tmux)
+    if s:supported_terminal == ''
         return 0
     endif