Cmd optional arg to not render terminal window

[BoogerMan2103]

Added new functionality: choice to draw a terminal window or not

In some cases it was needed to be rendered without terminal window, to show how something looks like on Linux but not MacOS

Tests:

task: [test] go test  -failfast -race -coverpkg=./... -covermode=atomic -coverprofile=coverage.txt ./... -run . -timeout=5m
?       github.com/mrmarble/termsvg/cmd/termsvg [no test files]
?       github.com/mrmarble/termsvg/cmd/termsvg/export  [no test files]
?       github.com/mrmarble/termsvg/cmd/termsvg/play    [no test files]
?       github.com/mrmarble/termsvg/cmd/termsvg/rec     [no test files]
?       github.com/mrmarble/termsvg/internal/testutils  [no test files]
?       github.com/mrmarble/termsvg/pkg/color   [no test files]
ok      github.com/mrmarble/termsvg/internal/svg        0.497s  coverage: 78.6% of statements in ./...
ok      github.com/mrmarble/termsvg/internal/uniqueid   0.529s  coverage: 100.0% of statements in ./...
ok      github.com/mrmarble/termsvg/pkg/asciicast       1.038s  coverage: 78.6% of statements in ./...
ok      github.com/mrmarble/termsvg/pkg/css     0.781s  coverage: 21.4% of statements in ./...

I am not a Go expert and of course all my changes may be rejected

[MrMarble]

Thank you for your contribution! I'm happy people use the tools I build for fun!