[enhancement] Merge multiple cast files

[warmchang]

Can we consider adding a merge subcommand?

merge: Merge multiple cast

An example, we can merge "1.cast" & "2.cast" to a new cast file.
According to the order of input files, "2.cast" is automatically appended to "1.cast":

./asciinema-edit merge --out ./mergefile.cast ./1.cast ./2.cast

/label enhancement

[abernier]

this is in my opinion a major feature to have +1