Restore include_template tag (#47)

This works with Fluid, and there is no reason to mandate a change;
we will document the different behavior with include

src/MyWebLog/Template.fs

@@ -316,10 +316,14 @@ open MyWebLog.Data
 module Cache =
 
     open System.Collections.Concurrent
+    open System.Text.RegularExpressions
 
     /// Cache of parsed templates
     let private _cache = ConcurrentDictionary<string, IFluidTemplate> ()
 
+    /// Custom include parameter pattern
+    let private hasInclude = Regex("""{% include_template \"(.*)\" %}""", RegexOptions.None, TimeSpan.FromSeconds 2L)
+
     /// <summary>Get a template for the given theme and template name</summary>
     /// <param name="themeId">The ID of the theme for which a template should be retrieved</param>
     /// <param name="templateName">The name of the template to retrieve</param>
@@ -336,8 +340,25 @@ module Cache =
             | Some theme ->
                 match theme.Templates |> List.tryFind (fun t -> t.Name = templateName) with
                 | Some template ->
-                    _cache[templatePath] <- parser.Parse(template.Text)
-                    return Ok _cache[templatePath]
+                    let mutable text = template.Text
+                    let mutable childNotFound = ""
+                    while hasInclude.IsMatch text do
+                        let child = hasInclude.Match text
+                        let childText =
+                            match theme.Templates |> List.tryFind (fun t -> t.Name = child.Groups[1].Value) with
+                            | Some childTemplate -> childTemplate.Text
+                            | None ->
+                                childNotFound <-
+                                    if childNotFound = "" then child.Groups[1].Value
+                                    else $"{childNotFound}; {child.Groups[1].Value}"
+                                ""
+                        text <- text.Replace(child.Value, childText)
+                    if childNotFound <> "" then
+                        let s = if childNotFound.IndexOf ";" >= 0 then "s" else ""
+                        return Error $"Could not find the child template{s} {childNotFound} required by {templateName}"
+                    else
+                        _cache[templatePath] <- parser.Parse(text)
+                        return Ok _cache[templatePath]
                 | None ->
                     return Error $"Theme ID {themeId} does not have a template named {templateName}"
             | None -> return Error $"Theme ID {themeId} does not exist"