Flag to set asset directory

[reccanti]

Create a compilation flag to set the asset directory location and name for HTML emails. The asset directory works fine for live previewing images right now, but when it comes to setting a compiled version to send off, more customizability might be needed.

Example implementation:

emailstack compile test.html -a .

This would compile test.html with assets loaded into the current directory. The default can still be a folder called assets in the output folder