Fix implementation of errorbar()

At the moment, the errorbar() function takes an optional parameter s,
which is presumably supposed to correspond to the same parameter for
e.g. the plot function. The only problem is that a) this argument is
unused in the function and b) matplotlib's errorbar() doesn't take a
parameter like this anyway.

I've replaced the s argument with an optional keywords argument, as for
other functions, which allows for setting colour, marker style etc.

matplotlibcpp.h

@@ -576,7 +576,7 @@ bool loglog(const std::vector<NumericX>& x, const std::vector<NumericY>& y, cons
 }
 
 template<typename NumericX, typename NumericY>
-bool errorbar(const std::vector<NumericX> &x, const std::vector<NumericY> &y, const std::vector<NumericX> &yerr, const std::string &s = "")
+bool errorbar(const std::vector<NumericX> &x, const std::vector<NumericY> &y, const std::vector<NumericX> &yerr, const std::map<std::string, std::string> &keywords = {})
 {
     assert(x.size() == y.size());
 
@@ -584,12 +584,15 @@ bool errorbar(const std::vector<NumericX> &x, const std::vector<NumericY> &y, co
     PyObject* yarray = get_array(y);
     PyObject* yerrarray = get_array(yerr);
 
-    PyObject *kwargs = PyDict_New();
+    // construct keyword args
+    PyObject* kwargs = PyDict_New();
+    for(std::map<std::string, std::string>::const_iterator it = keywords.begin(); it != keywords.end(); ++it)
+    {
+        PyDict_SetItemString(kwargs, it->first.c_str(), PyString_FromString(it->second.c_str()));
+    }
 
     PyDict_SetItemString(kwargs, "yerr", yerrarray);
 
-    PyObject *pystring = PyString_FromString(s.c_str());
-
     PyObject *plot_args = PyTuple_New(2);
     PyTuple_SetItem(plot_args, 0, xarray);
     PyTuple_SetItem(plot_args, 1, yarray);
@@ -766,7 +769,7 @@ inline void figure_size(size_t w, size_t h)
     PyDict_SetItemString(kwargs, "figsize", size);
     PyDict_SetItemString(kwargs, "dpi", PyLong_FromSize_t(dpi));
 
-    PyObject* res = PyObject_Call(detail::_interpreter::get().s_python_function_figure, 
+    PyObject* res = PyObject_Call(detail::_interpreter::get().s_python_function_figure,
             detail::_interpreter::get().s_python_empty_tuple, kwargs);
 
     Py_DECREF(kwargs);
@@ -890,7 +893,7 @@ inline void xticks(const std::vector<Numeric> &ticks, const std::vector<std::str
     Py_DECREF(args);
     Py_DECREF(kwargs);
     if(!res) throw std::runtime_error("Call to xticks() failed");
-    
+
     Py_DECREF(res);
 }
 
@@ -937,7 +940,7 @@ inline void yticks(const std::vector<Numeric> &ticks, const std::vector<std::str
     Py_DECREF(args);
     Py_DECREF(kwargs);
     if(!res) throw std::runtime_error("Call to yticks() failed");
-    
+
     Py_DECREF(res);
 }