Awsume

- Properly returns None as expiration for credentials without expiration

- Supports awsuming role profiles who's `source_profile` reference another role profile, repeatedly
- [98](https://github.com/trek10inc/awsume/pull/98) Exports role credential expiration time to environment variables

- Supports exporting awsume'd credentials to the profile specified by the `-o`/`--output-profile` command-line flag
- Works closely with autoawsume
- If specifying `-a` and `-o` in the same command, the created autoawsume profile will be under the output profile name from the `-o` flag
- Adds a `--clean` flag to clean up expired output profiles
- Exports `AWSUME_COMMAND` with arguments passed to awsume command on a successful execution

- Will export AWS_PROFILE when awsuming a profile with `credential_source` set to `Environment`, `Ec2InstanceMetadata`, or `EcsContainer`

- Breaks `xmltodict` and `python-levenshtein` out into extras_require (so you'll have to `pip install awsume[fuzzy]` to get the fuzzy-match featureset which requires `python-levenshtein`. The same applies to `saml` and `xmltodict`)

- [95](https://github.com/trek10inc/awsume/issues/95) Adds support for `credential_source` as `Ec2InstanceMetadata` and `EcsContainer`