math preview: option to use preamble & environments of the current .tex file

[ghost]

Hi, thanks a lot for the new math/figure preview feature! However, as has been reported in #915, if my current latex document has some custom preamble or environment, e.g. \newcommand{\Prob}{\mathbf{P}} \usepackage{braket} then the preview for equations containing these symbols are not generated properly. In #915, this is classified as 'performance issue' and references to #896. However, the solution in that issue is not very good: one has to provide a custom preamble, almost hard-coded in the preference file or link to another file. How about just use the preamble and environments defined in the current .tex file? Thanks a lot!