Improve IRuby display

[isuruf]

@zverok wrote,

Is this difference between text/html and text/plain ok? As far as I can understand, IRuby prefers to output text/html, when finds it, and it completely hides the fact that we are working with SymEngine object.

BTW, I like to do HTML representation of such kind of objects this way (look on blue-ish entities, and hover the mouse over them): basic text is as simple as possible, background+border shows "it is something more complex", tooltip shows inspect results.

See https://github.com/symengine/symengine.rb/pull/59#r69952644