text: Title: Percent in Axis label produces hard to read error. need to escape '%' or Warning?
  Body: Regarding #50 
  
  When assigning a axis label, I was describing a percent, and used the percent symbol. Everything was produced fine in python, and went as expected.
  
  However, I could not figure out why it kept crashing in latex a week later. It took me around an hour and a half, as the latex error output was not helpful at all
  
  Here's reproducible code
  
  ```
  import pandas as pd
  import matplotlib.pyplot as plt
  from matplotlib2tikz import save as tikz_save
  
  
  #RANDOM DATA
  des = [1,2,3,4]
  perc_below = [1,2,3,4]
      
      
  
  df3 = pd.DataFrame({'one':perc_below, 'two':des})
  
  df3.plot(fontsize=20, kind='line',y='one',x='two')
  plt.xlabel('example',fontsize=20)
  
  # ISSUE LIES BELOW
  plt.ylabel('example %',fontsize=20)
  
  tikz_save('example.tex')`
  
  
  
  ```
  
  Tex output is as expected...
  
  In my opinion, this should be escaped, as the work around is a backslash when assigning the label. However, this would lead to different behavior as matplotlib. (I also export to pngs, which would end up with a backslash in them)
  
  I understand if you do not agree. In that case, a warning when the graph is produced would be great, as again, the latex error was completely unhelpful.
  
  I love the library, BTW!