Add support for "%s" in qcc_query_command

Squashed commit of the following:

commit e0f3a6f
Author: Mark Stillwell <marklee@fortawesome.org>
Date:   Tue Dec 25 22:25:08 2012 +0000

    update, removed multiline feature temporarily, added support for %s in
    g:qcc_query_command.

commit ab773fa
Author: Mark Stillwell <marklee@fortawesome.org>
Date:   Sun Dec 23 21:09:41 2012 +0000

    fixed regexp to get query command from mutt and added support for multi-line
    address fields.

Ref: #2

plugin/querycommandcomplete.vim

@@ -51,7 +51,7 @@ endif
 " use mutt query command as default
 if !exists("g:qcc_query_command")
     let s:querycmd = system('mutt -Q query_command 2>/dev/null')
-    let s:querycmd = substitute(s:querycmd, '^query_command=\"\(.*\) .%s.\"\n', '\1','')
+    let s:querycmd = substitute(s:querycmd, '^query_command="\(.*\)"\n', '\1','')
 
     if len(s:querycmd)
         let g:qcc_query_command = s:querycmd
@@ -107,12 +107,12 @@ function! s:GenerateCompletions(findstart, base)
         return s:FindStartingIndex()
     endif
 
-    if a:base =~ '^ *$'
-        return []
-    endif
-
     let results = []
-    let cmd = g:qcc_query_command . ' ' . shellescape(a:base)
+    let cmd = g:qcc_query_command
+    if cmd !~ '%s'
+        let cmd .= ' %s'
+    endif
+    let cmd = substitute(cmd, '%s', shellescape(a:base), '')
     let lines = split(system(cmd), g:qcc_line_separator)
 
     for my_line in lines
@@ -139,10 +139,7 @@ function! s:GenerateCompletions(findstart, base)
 endfunction
 
 function! QueryCommandComplete(findstart, base)
-    let cur_line = getline(line('.'))
-
-    " TODO: Figure out a way to handle multiline
-    if cur_line =~ g:qcc_pattern
+    if getline('.') =~ g:qcc_pattern
         return s:GenerateCompletions(a:findstart, a:base)
     endif
 endfunction