Max value font size in solidgauge charts is too small and not configurable

The solidgauge charts display the value as specified in 'max_value'. However, the rendered PNG has too small a font for the maximum value.
There is no such option to configure the font size of the same.

`import pygal`
`args = {
"half_pie": True,
"inner_radius": 0.70,
"formatter": lambda x: '{0} GiB'.format(x)
}`
`chart = pygal.SolidGauge(**args)`
`chart.add("Serie", 30)`
`chart.render_to_png('gauge.png')`

Output

You should be able to observe a tiny font with a magnifying glass :-)

Start with pygal.SolidGauge and follow the render_to_png('gauge.png') path for the displayed max_value. Determine where the maximum value's font size is set and how chart options are handled. Done means the maximum value is legible in the rendered PNG and its font size can be configured.

Written by the indexing model from the issue text.