Add Azure Blob Storage replica type

[benbjohnson]

Overview

This pull request implements an Azure Blob Storage (ABS) replica.

Fixes #134

Usage

To use it, specify a replica URL with a type of "abs". The URL host represents the container name and the URL path represents the path within the container:

litestream replicate /path/to/db abs://CONTAINERNAME/db

To use in a config, you can use the URL:

dbs:
  - path: /path/to/db
    replicas:
      - url: abs://mycontainer/db

or split the parts into separate fields:

dbs:
  - path: /path/to/db
    replicas:
      - type: abs
        bucket: mycontainer
        path: db

You can also specify the endpoint field when using the config, however, you typically shouldn't need to.