Support lte, gte filtering on attributes

First of all, thank you for the work you've done on this!

For a task with a custom UDA such as project duration (`duration`) in man-days, or number of resources (`resources`), I think it would be helpful to filter those tasks in a Django ORM-like manner.

Example of the API:
```
from tasklib import TaskWarrior
tw = TaskWarrior(data_location="~/vaults/tasks")
tw.tasks.pending().get(duration__gte=7)
```
This will query any pending tasks with a duration value of `7` or more.

This is very consistent to the current behavior as [documented officially]( https://tasklib.readthedocs.io/en/latest/#task-attributes):
```
tw.tasks.pending().get(tags__contain='work')
```

Currently, the work-around is to read all tasks in a for-loop and then do a conditional check, but I thought this would be a much appreciated feature. What do you guys think?