Automatically load exported style names

[jonas0616]

In the tutorial, we can change the output style via set_style(), and MSWORD_FRIENDLY is one of the styles which can be imported as following:

from prettytable import MSWORD_FRIENDLY

It also mentioned there are another styles, e.g., PLAIN_COLUMNS. However, it failed to import via the same way.

from prettytable import PLAIN_COLUMNS

result:

Traceback (most recent call last):
  File "./pptest.py", line 3, in <module>
    from prettytable import PLAIN_COLUMNS
ImportError: cannot import name PLAIN_COLUMNS

I am unable to solve it until tracing the source code, and change to the following import statement:

from prettytable.prettytable import PLAIN_COLUMNS

I am of the opinion that it should automatically load all exported styles instead of only partial ones. If not, it should then add some notes in tutorial to clarify the usage.

[coveralls]

Coverage increased (+0.02%) to 77.488% when pulling dbbeb25 on jonas0616:export-style-name into 4676e41 on kxxoling:master.

[hugovk]

Tip: this is fixed in upstream PrettyTable.

https://github.com/jazzband/prettytable