Fix partials to indent as demanded by the mustache spec

The mustache specs [1] and testsuite [2], as well as the lustache README.md [3]
 demand that partials be indented.

[1]: http://mustache.github.io/mustache.5.html
[2]: https://github.com/mustache/spec/blob/v1.1.3/specs/partials.yml#L82-L100
[3]: https://github.com/Olivine-Labs/lustache#partials

src/lustache/renderer.lua

@@ -64,7 +64,7 @@ local function compile_tokens(tokens, originalTemplate)
         t == "^" and rnd:_inverted(
           token.value, ctx, subrender(i, token.tokens)
         ) or
-        t == ">" and rnd:_partial(token.value, ctx, originalTemplate) or
+        t == ">" and rnd:_partial(token.value, ctx, originalTemplate, token.startIndex) or
         (t == "{" or t == "&") and rnd:_name(token.value, ctx, false) or
         t == "name" and rnd:_name(token.value, ctx, true) or
         t == "text" and token.value or ""
@@ -249,7 +249,7 @@ function renderer:_inverted(name, context, callback)
   return ""
 end
 
-function renderer:_partial(name, context, originalTemplate)
+function renderer:_partial(name, context, originalTemplate, startIndex)
   local fn = self.partial_cache[name]
 
   -- check if partial cache exists
@@ -264,7 +264,23 @@ function renderer:_partial(name, context, originalTemplate)
     fn = self:compile(partial, nil, originalTemplate)
     self.partial_cache[name] = fn
   end
-  return fn and fn(context, self) or ""
+
+  local startOfLineIndex = 1
+  for i = startIndex, 1, -1 do
+    if string_sub(originalTemplate, i, i) == "\n" then
+      startOfLineIndex = i + 1
+      break
+    end
+  end
+  local indent = string_sub(originalTemplate, startOfLineIndex, startIndex - 1)
+
+  local str = fn and fn(context, self) or ""
+  local buf = {}
+  for i,line in ipairs(string_split(str, "\n")) do
+    buf[#buf+1] = (i > 1 and indent or "") .. line .. "\n"
+  end
+
+  return table_concat(buf)
 end
 
 function renderer:_name(name, context, escape)