Add a tag option in the extract program to optionally give an opportunity to tailor some packages in for example the stdlib, for people who is interested in keeping the final binary size as smaller as possible.
Below go generate
```
//go:generate ../cmd/extract/extract -name stdlib -tag stdmime mime mime/multipart mime/quotedprintable
```
produces a header to stdlib/mime-multipart.go
```
// Code generated by 'yaegi extract mime/multipart'. DO NOT EDIT.
// +build go1.16,!go1.17,stdmime
```
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