Support Excel Title Cell Formatting

Adding support for merging and aligning title cells in Excel.  Example:

exportService.export(params.formatName, response, "${project.name} -
Vendor List", params.extension, project.vendors as List, fields,
labels, [:], [titles:["${project.name} - Vendor List"],
'titles.mergeCells': true, 'titles.alignment': 'CENTRE'])