Convert project_name to lowercase

```bash
$ fig up
Creating FIGtest_test_1...
Building test...
Invalid repository name (FIGtest_test), only [a-z0-9-_.] are allowed
$ cat fig.yml 
test:
  build: test/
$ cat test/Dockerfile 
FROM scratch
$ basename $(pwd)
FIG-test
```

IMHO `.` and `-` could be allowed in the a `normalize_name` method too. What is your opinion?