Add buffer var for main file (fixes #155)

autoload/vimtex.vim

@@ -168,6 +168,13 @@ function! s:get_id(main) " {{{1
 endfunction
 
 function! s:get_main() " {{{1
+  "
+  " Use buffer variable if it exists
+  "
+  if exists('b:vimtex_main') && filereadable(b:vimtex_main)
+    return fnamemodify(b:vimtex_main, ':p')
+  endif
+
   "
   " Search for main file specifier at the beginning of file.  This is similar
   " to the method used by several other plugins and editors, such as vim with

doc/vimtex.txt

@@ -150,6 +150,13 @@ first five lines of the current file similar to this: >
 
   %! TEX root = my-main.tex
 
+<                                                                *b:vimtex_main*
+Finally, the main file may be specified through the buffer variable
+`b:vimtex_main`.  If one uses project specific |vimrc| files, one may then use
+an |autocmd| to specify the main file through this buffer variable with >
+
+  autocmd FileType tex let b:vimtex_main = 'main.tex'
+
 ------------------------------------------------------------------------------
 Comment on internal tex plugin~
                                                       *vimtex-comment-internal*