Enhances the display of the list of virtualenvs

This proposes to enhance the displaying of the list of virtualenvs by sorting and aligning them by columns.

For example, instead of :
```
aioa billing blogblan bond dailyccoop erp erp8 eshop fab fdt helpdesk pelican pew photoblan pip-accel prestapyt pyodbc radicale rshare sentry test test2 travis tryton weasyprint
```
You would have :
```
aioa      dailyccoop  fab       pew        pyodbc    test    weasyprint
billing   erp         fdt       photoblan  radicale  test2
blogblan  erp8        helpdesk  pip-accel  rshare    travis
bond      eshop       pelican   prestapyt  sentry    tryton
```
It's more readable...

If you pipe `pew workon` to a file or another process, we print it with a single virtualenv by line.