Add LEIN_SILENT option to supress *info* output

If a LEIN_SILENT environment variable is set as truthy, then *info* will be set to false and output will be supressed. See #1387. Default is still true since LEIN_SILENT will return `nil` if not set.

Examples (in bash):

```bash
$ LEIN_SILENT=true lein pom` 
=> No output
```

Note that, because `System/getenv` returns a string, any string value will return truthy including `LEIN_SILENT=false`:

```bash
$ LEIN_SILENT=false lein pom` 
=> No output
```

You should erase the `LEIN_SILENT` variable to return to the default behavior:

```bash
$ set LEIN_SILENT
$ lein pom
=> Wrote /path/to/pom.xml
```