Added support for custom payload transforms

[danielmorell]

Description of the change

This PR adds support for custom transforms. It also specifies how transforms are ordered based on a priority attribute. It retains the current transform order but now allows a user to insert a custom transform at any point in the transform stack.

The custom transform use would look something like this:

import rollbar
from rollbar.lib.transforms import Transform

class MyTransform(Transform):
    # A lower priority is value is sooner.
    # Built-in transforms are set at 10, 20, 30, 40, and 50.
    # With a priority of 100, this will run last.
    priority = 100

    # This method will be called for each item in the payload. 
    # See rollbar.lib.transforms.Transform for specific methods for most types.
    def default(self, o, key=None):
        return o


# custom_transforms should be a list of Transform objects.
rollbar.init('<my secret key>', 
             custom_transforms=[MyTransform()])

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

• Fix Ability to transform payload #245

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• [x All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer