Add \text{} command

[ForNeVeR]

Reported by our user in #82 (comment): \text{abc} doesn't work; it says "Unknown symbol or command or predefined TeXFormula: 'text'".

I've confirmed that \text{} command never worked in any released version, so that's not a regression but a lack of feature.

[alexreg]

Oh wow, my bad. I thought it did!

[abozanona]

I hope that the \text{} command will work for all UTF-8 characters. @ForNeVeR Could you tell how much time would it take to add this feature?

[ForNeVeR]

@abozanona as usual in open source, don't consider anyone here (myself included) to make any strong obligations. Although personally I wanted to start implementing this task in the nearest couple of days. I have no idea how long will it take, but I think it's reasonable to expect that it will be ready on next Monday (with about 65% certainty).

I consider this feature as a separate one from #82 (UTF-8 support), but I'll do my best to also somehow solve #82 in the same time frame.