feat: allow --output to override output location

[maaslalani]

Allow Output location to be overridden by CLI flags:

Specify multiple outputs in the same flag.

vhs demo.tape --output out.gif,out.webm

Pass multiple flags (with shorthand):

vhs demo.tape -o out.gif -o out.webm

Note: this will override any Output commands set in the tape file.

[maaslalani]

cc @mxcl