Trying to write a more robust TemplateEngine with replacements defined externally

Question

I am in the process of writing a TemplateEngine. The intention is that will read in a HTML file containing placeholders. Something along the lines of <p><Dear |*name|* you have won 1st prize in |*competition|* thanks for your entry on |*date|*</p> where anything between || needs to be replaced by data from a key value pair dictionary.

I currently initialise a dictionary manually like this:

mergeData.Add("name", dataRow["UserName"].ToString());
mergeData.Add("competiton", dataRow["CompName"].ToString());
mergeData.Add("date", dataRow["EntryDate"].ToString());
templateEngine.Initialise(mergeData);
templateEngine.Run();`

As you can see it makes use of a few magic strings. What I would like to do is make it a bit more extensible so that the placeholders and replacements could be defined externally. At the moment I am explictly specifying which columns to use for the data in code. I think that maybe a DataTable would work for the Initialise() but not sure how to approach it. Any ideas / suggestions would be most welcome.

    public class TemplateEngine
    {
        private string _template;
        private bool _initialised = false;
        private Dictionary<string, string> _mergeData;

        private TemplateEngine(string templateString)
        {
            _template = templateString;
        }

        public void Initialise(Dictionary<string, string> mergeData)
        {
            if (mergeData == null)
            {
                _initialised = false;
                throw new ArgumentException("Must specify key value pairs to perform merge correctly");
            }
            _mergeData = mergeData;
            _initialised = true;
        }

        public string Run()
        {
            if (_initialised == false)
            {
                throw new Exception("Cannot run engine as mergeData dictionary is not initalised");
            }
            foreach (var kvp in _mergeData)
            {
                _template = _template.Replace("|*" + kvp.Key + "|*", kvp.Value);
            }
            return _template;
        }

        public static TemplateEngine FromFile(string filePath)
        {
            if (filePath == string.Empty)
            {
                throw new ArgumentException("FilePath not specified cannot instantiate TemplateEngine");
            }
            string html = System.IO.File.ReadAllText(filePath);
            var templateEngine = new TemplateEngine(html);
            return templateEngine;
        }
    }
}