Add `--format` cli option, defaults to Vim format

To match the behavior of ctags, the default format is now Vim, which goes along well with the default output file `./tags`. If the output file is changed to `TAGS`, Emacs format is automatically triggered.

This makes `ripper-tags -R` a compatible replacement for ctags in a Ruby project.

User can use the `--format` option to explicitly choose between "vim", "emacs", "json" and "custom" (our DefaultFormatter).

```
Usage: ripper-tags [options] FILES...

    -f, --tag-file (FILE|-)          File to write tags to (default: `./tags')
                                     "-" outputs to standard output
    -R, --recursive                  Descend recursively into subdirectories
        --all-files                  Parse all files as ruby files, not just `*.rb' ones
 
        --format (emacs|json|custom) Set output format (default: vim)
    -e, --emacs                      Output Emacs format (default if `--tag-file' is `TAGS')

    -d, --debug                      Output parse tree
        --debug-verbose              Output parse tree verbosely
    -V, --verbose                    Print additional information on stderr
        --force                      Skip files with parsing errors
    -v, --version                    Print version information
```