`mpl_to_plotly` does not preserve axis labels (bar plots are useless)

While mpl_to_plotly is little known and receives little love, this bug is pretty easy to fix. Would you be open to a PR?

import matplotlib.pyplot as plt
from plotly.tools import mpl_to_plotly
from plotly.offline import plot

In matplotlib this produces a barplot with labels:

labels = ['a', 'b', 'c']
values = [1, 2, 3]

f = plt.figure(figsize=(6, 4))
plt.bar(labels, values)
plt.tight_layout()

But conversion to plotly looses the labels:

plotly_fig = mpl_to_plotly(f)
plot(plotly_fig)

A minimal fix would be to modify prep_ticks by appending:

    if axis_dict.get("type") == "date":
        return axis_dict
    
    vals = []
    texts = []
    for tick in axis.majorTicks:
        vals.append(tick.get_loc())
        texts.append(tick.label1.get_text())
    
    if texts:
        axis_dict = {}
        axis_dict['tickmode'] = 'array'
        axis_dict['tickvals'] = vals
        axis_dict['ticktext'] = texts
    
    return axis_dict

which produces:

prep_ticks is defined in:

https://github.com/plotly/plotly.py/blob/c54a2bdf1655caaaa3b7b71fbfc38a5584767bd5/plotly/matplotlylib/mpltools.py#L428-L514

Start in plotly/matplotlylib/mpltools.py at prep_ticks, then reproduce the supplied matplotlib bar plot and mpl_to_plotly conversion. Preserve the original axis labels in the converted Plotly figure, and verify the resulting plot matches the demonstrated labeled output without regressing date-axis handling.