Simplify and document the config key for toggling language defaults

Closes preservim#33

README.md

@@ -83,6 +83,9 @@ let g:NERDCompactSexyComs = 1
 " Align line-wise comment delimiters flush left instead of following code indentation
 let g:NERDDefaultAlign = 'left'
 
+" Set a language to use its alternate delimiters by default
+let g:NERDAltDelims_java = 1
+
 " Add your own custom formats or override the defaults
 let g:NERDCustomDelimiters = { 'c': { 'left': '/**','right': '*/' } }
 ```

doc/NERD_commenter.txt

@@ -751,11 +751,11 @@ again.
 
 If you want the NERD commenter to use the alternative delimiters for a
 specific filetype by default then put a line of this form into your vimrc: >
-    let g:NERD_<filetype>_alt_style=1
+    let g:NERDAltDelims_<filetype> = 1
 <
 Example: java uses // style comments by default, but you want it to default to
 /* */ style comments instead. You would put this line in your vimrc: >
-    let g:NERD_java_alt_style=1
+    let g:NERDAltDelims_java = 1
 <
 
 See |NERDComAltDelim| for switching commenting styles at runtime.

plugin/NERD_commenter.vim

@@ -516,7 +516,7 @@ function s:SetUpForNewFiletype(filetype, forceReset)
         endfor
         " if g:NERD_<filetype>_alt_style is defined, use the alternate style
         let b:NERDCommenterFirstInit = getbufvar(1,"NERDCommenterFirstInit",0)
-        if exists('g:NERD_'.ft.'_alt_style') && eval('g:NERD_'.ft.'_alt_style') && !b:NERDCommenterFirstInit
+        if exists('g:NERDAltDelims_'.ft) && eval('g:NERDAltDelims_'.ft) && !b:NERDCommenterFirstInit
             call s:SwitchToAlternativeDelimiters(0)
             let b:NERDCommenterFirstInit = 1
         endif