Implement Detect

As the comment suggested, use github.com/ryanuber/go-license.

For this, to recognize MPL2, licenses/mpl2.header needed a small
change: instead of "v. 2.0", now it contains "version 2.0".
Hope this is ok.

For set, I've added a detectLicense + removeLicense before the
insertLicense, to mitigate license duplication.
This does work for licenses without blank lines, but for more
comples header formats, as only the first paragraph is removed.

So mpl2 -> gpl2 ok, but not gpl2 -> mpl2.
insertLicense: replace @@owner@@ only in copyright & license header, not in the file body

Add "\n" only in insertLicense, and only after the copyright and
the license headers.
Do not separate the copyright and license body with an empty line,
but use the EOLCommentStyle.

Do not require *Config in prependEOLComment, as only the EOLCommentStyle
is required there.
Add --replace option to set.

Only try to replace old license with the new one if it is asked for
by specifying the "--replace" option.
Merge branch 'tgulacsi-master'