Issue title: Error: Can not find file mysql://...
 Issue body: Version: 3.1.0

try
```bash
user@localhost:~$ pgloader -D ./pgloader --debug mysql://user:pass@localhost:3306/db1 postgresql://user:wwalker@example.org:5432/db1
```
result
```bash
sb-impl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5*default-external-format* :UTF-8
tmpdir: #P"/tmp/pgloader/"
2017-11-29T08:06:47.006000Z LOG Starting pgloader, log system is ready.
2017-11-29T08:06:47.014000Z INFO Starting monitor
2017-11-29T08:06:47.017000Z LOG Main logs in '/home/user/pgloader/pgloader.log'
2017-11-29T08:06:47.017000Z LOG Data errors in '/home/user/pgloader/'
2017-11-29T08:06:47.017000Z ERROR Can not find file: "mysql://user:pass@localhost:3306/db1 "
2017-11-29T08:06:47.017000Z ERROR Can not find file: "postgresql://user:wwalker@example.org:5432/db1"
2017-11-29T08:06:47.017000Z INFO Stopping monitor
```
Why "Can not find file" if i use connection string?
 Comments: 
 Comment 0: You're using a version where the only supported command arguments where still load command files. Please upgrade!
{'source': 'GithubIssues', 'repo_name_with_owner': 'dimitri/pgloader', 'issue_id': 277663430, 'repo_stars': 4247.0}