Add Github Flavored Markdown table export format (--gfm)

Export article data in [Github Flavored Markdown] (https://help.github.com/articles/github-flavored-markdown) table syntax.

```
./scholar.py --gfm -c 1 -a "Peter Norvig"

| Title          | A modern approach                                                                         |
| -------------- | ----------------------------------------------------------------------------------------- |
| Year           | 1995                                                                                      |
| Citations      | 23127                                                                                     |
| Versions       | 60                                                                                        |
| Cluster ID     | 16379301881339907933                                                                      |
| Citations list | http://scholar.google.com/scholar?cites=16379301881339907933&as_sdt=2005&sciodt=0,5&hl=en |
| Versions list  | http://scholar.google.com/scholar?cluster=16379301881339907933&hl=en&as_sdt=0,5           |
```

This text is also parsed in Github Flavored Markdown, so using the previous code in here we get:

| Title          | A modern approach                                                                         |
| -------------- | ----------------------------------------------------------------------------------------- |
| Year           | 1995                                                                                      |
| Citations      | 23127                                                                                     |
| Versions       | 60                                                                                        |
| Cluster ID     | 16379301881339907933                                                                      |
| Citations list | http://scholar.google.com/scholar?cites=16379301881339907933&as_sdt=2005&sciodt=0,5&hl=en |
| Versions list  | http://scholar.google.com/scholar?cluster=16379301881339907933&hl=en&as_sdt=0,5           |