Do not read temporary editor files, fixes #548

This fixes a bug where Terraform would error with the following:

```
Error loading config: Error reading
/Users/rhenrichs/work/example/.#example.tf: open
/Users/rhenrichs/work/example/.#example.tf: no such file or directory
```

The solution implemented here ignores the common emacs and vim
temporary file formats.

Note: the potential danger with merging this is that Terraform could
quickly have requests to ignore other file formats.
Merge pull request #632 from KushalP/master

config: do not read temporary editor files, fixes #548