Export instances of supported MIME types to make checking easier

[bouwerp]

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
...

[bouwerp]

B.t.w., how do I submit a PR to add this?