[ENH] Define cl.Styler class #1332

I believe we should start by adapting the existing pd.io.formats.style.Styler and then gradually augmenting it with support for actuarial elements (lower triangle, diagonals, etc.).

With #1331 being such an ambitious idea, this may be a case of perfection being the enemy of good. Maybe one day we'll have a 100% custom in-house Styler, but this a very complex class. Since Triangles are printed as DataFrames anyway, I believe starting with the Pandas Styler is appropriate.