Release file: support Version field

https://wiki.debian.org/DebianRepository/Format#Version

The Version field, if specified, shall be the version of the release.
On the other hand, if not set or set to an empty value, the Version
field will not be included in the Release file.

Signed-off-by: Zhang Xiao <xiao.zhang@windriver.com>

man/aptly.1

@@ -1569,6 +1569,10 @@ Options:
 set value for Signed-By field
 .
 .TP
+\-\fBversion\fR
+version of the release
+.
+.TP
 \-\fBacquire\-by\-hash\fR
 provide index files by hash
 .
@@ -1714,6 +1718,10 @@ Options:
 set value for Signed-By field
 .
 .TP
+\-\fBversion\fR
+version of the release
+.
+.TP
 \-\fBacquire\-by\-hash\fR
 provide index files by hash
 .
@@ -2187,6 +2195,10 @@ Options:
 set value for Signed-By field
 .
 .TP
+\-\fBversion\fR
+version of the release
+.
+.TP
 \-\fBbatch\fR
 run GPG with detached tty
 .