tasklib should respect context or support it as an option IMO

I think context should be supported.

As a workaround I have code similar to this:

```python3
from tasklib import TaskWarrior

tw = TaskWarrior(data_location='~/.task')

context = tw.execute_command(["_get","rc.context"])[0]

if context:
context_read = tw.execute_command(["_get","rc.context." + context + ".read"])[0]
context_filter = '+PENDING and ( ' + context_read + ')'
nexttasks = tw.tasks.filter(context_filter)
else:
nexttasks = tw.tasks.pending()

```