Add option to specify a custom location for report generation

### Changes

- renamed `constructor` in `bin/xcpretty` to `printer_opts`
- added `report_options` to `bin/xcpretty` to handle custom report path argument
- use path option in JUnit reporter if specified, otherwise defaults to original location at `build/reports/junit.xml`
- move `bin/xcpretty` helper methods into `XCPretty::Runner` class

### Note

This is a breaking change to reporter functionality. If anyone has a custom reporter, the constructor now takes an options hash.


Fixes #42