add option to specify lavaan syntax for fit_uni_lcsm() and fit_bi_lcsm()

[milanwiedemann]

this will be helpful if there are specific specifications needed for a particular model that cant be generated by the functions of this package

[milanwiedemann]

not sure if this is helpful, at the moment I can return the lavaan syntax for a specified model, then make manual changes and just use the lavaan function instead of fit_uni_lcsm()