templater: factor out function that creates templater from string template This function will host loading of template aliases. It is not defined at templater, but at formatter, since formatter is the module handling ui stuff in front of templater.

templater: separate function to create templater from map file (API) New frommapfile() function will make it clear when template aliases will be loaded. They should be applied to command arguments and templates in hgrc, but not to map files. Otherwise, our stock styles and web templates (i.e map-file templates) could be modified unintentionally. Future patches will add "aliases" argument to __init__(), but not to frommapfile().