Add support for axes.Axes._tikzplotlib_anchors

[st--]

The documentation mentions support for a _tikzplotlib_anchors attribute on matplotlib Axes() objects to add custom anchors in the plot (very useful to refer back to from custom tikz code outside the auto-generated one, e.g. to add additional content from the outside). However, this isn't actually supported in the code. This PR adds a minimal implementation; I'd be happy to discuss how to improve it!

Example Python code:

import matplotlib.pyplot as plt
plt.plot([1,2,3], [4,-2,3])
ax = plt.gca()
ax._tikzplotlib_anchors = [((1.5, 2.5), "foo")]
import tikzplotlib as tikz
tikz.save("test.tex")

Resulting generated output:

% This file was created with tikzplotlib v0.10.1.
\begin{tikzpicture}

\definecolor{darkgray176}{RGB}{176,176,176}
\definecolor{steelblue31119180}{RGB}{31,119,180}

\begin{axis}[
tick align=outside,
tick pos=left,
x grid style={darkgray176},
xmin=0.9, xmax=3.1,
xtick style={color=black},
y grid style={darkgray176},
ymin=-2.3, ymax=4.3,
ytick style={color=black}
]
\addplot [semithick, steelblue31119180]
table {%
1 4
2 -2
3 3
};
\coordinate (foo) at (1.5,2.5);
\end{axis}

\end{tikzpicture}

Note the additional \coordinate (foo) at (1.5,2.5); line.

[st--]

NB- I'm not sure about calling it _tikzplotlib_anchors, I just went with that as it was in the docstring, but given that it's not implemented and no-one's been able to use it before, how about calling it _tikzplotlib_coordinates or _tikzplotlib_additional_coordinates instead?