Configurable test-data for shed resolution.

Add a `test_data` directive to `.shed.yml` with the obvious defaults.

```
test_data: ; totally optional, compeltely inferred from IUC "best practices"
auto: false ; inferring test data has some corner cases and would be off by default I guess
; and this dirctory would just be auto detected for "best practices" repos
include:
- 1.fasta ; manually specify tool-data contents if ``auto_locs: False``
base: ../../test-data
```

Useful to custom repository layouts (See #160) but also for de-multiplexing tool directories.

Start with the proposed `.shed.yml` structure and issue #160, then find the existing shed-resolution and configuration entry points. Done means supporting the optional `test_data` directive, its `auto`, `include`, and `base` values, and the stated defaults for custom repository layouts and de-multiplexed tool directories.

Written by the indexing model from the issue text.