Index changes and bug fixes

• Diagnostic functions now support section = :section_name as keyword arguments (#54)
• Corner plots have been fixed to work with multiple chains (#55)
• chn[:param] and chn[:, :param, :] now both return the same thing, a Chains object containing only :param.
• Strings are now enforced as parameter names. Symbols no longer work as parameter names. (#57)
• Added get_params, which packages all the parameters in a chain into a NamedTuple. Used like get_params(chn). (#57)
• Added an overload on get accepting a keyword section, which can be a vector or a single symbol. Works like get(chn; section = :pooled) or get(chn; section = [:parameters, :pooled]). (#57)
• All variants of get and get_params accept a keyword argument flatten, which can be true or false. If flatten=true, the tuple returned will have a flat structure. This is handy if you want to go straight to a dataframe, because flatten=false (the default) will clump variables with the same names together (as with "P[1]", "P[2]", "P[3]"). (#57)
• Fixed bug where chains would not show if their length was less than 200 (c2d0827)
• Fixed bug where chains would not show if they did not contain a :parameters section