Overloading the text() method to accept a keyword arguments map #372

I had a need for this in a local project which required loads of custom text elements: made a minimal working example based on the same keyword approach already used e.g. to overload the plot() method in matplotlibcpp.h, this seems to work ok for our purposes:

matplotlibcpp.h addition:
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...
void text(Numeric x, Numeric y, const std::string& s = "", const std::map<std::string, std::string>& keywords = {})
{ ...

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example usage:
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...
plt::text(x+.02125,y_val+.0075,string(pretty_bps),{{"color",EUREX_BLUE},{"fontsize","xx-small"},{"horizontalalignment","right"}});
...

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