Improve formula evaluation. Fix #201

A past change added support for html comments for tables, however, this
fails when the formula line happens to match the format of html comments

autoload/tablemode/spreadsheet/formula.vim

@@ -1,12 +1,12 @@
 " Private Functions {{{1
-function! s:IsHTMLComment(line) "{{{2
-  return getline(a:line) =~# '^\s*<!--'
-endfunction
-
 function! s:IsFormulaLine(line) "{{{2
   return getline(a:line) =~# 'tmf: '
 endfunction
 
+function! s:IsHTMLComment(line) "{{{2
+  return !s:IsFormulaLine(a:line) && getline(a:line) =~# '^\s*<!--'
+endfunction
+
 " Public Functions {{{1
 function! tablemode#spreadsheet#formula#Add(...) "{{{2
   let fr = a:0 ? a:1 : input('f=')