Export instances of supported MIME types to make checking easier

Export instances of usable MIME types, e.g.:
```
...
mType := mimetype.Detect(data)

if mimtype.XLSX.Is(mType.String()) {
    ...
}
...
```
This can easily be done by declaring exported variables dereferencing the values defined in _tree.go_:

```
var Xz = *xz
var GZIP = *gzip
var SEVENZ = *sevenZ
var ZIP = *zip
var TAR = *tar
...
``` 