add argument loading

[couleurm]

added second idea from #24

I could've probably made it simpler by using &str but it involves using lifetimes and I haven't learned that yet, if you consider accepting feel free to brush off the rough corners of my addition (gave you push access to my branch).

[quietvoid]

@couleurm Can you please test the changes.
The variables can be set with multiple -v args or a list delimited by ,.

vspreview-rs -v key=value -v ...
vspreview-rs -v key1=value1,key2=value2,...

[couleurm]

hey, it'd be cool to stick to VSPipe's conventions

• add aliases (-a and --arg) and support to use --arg var1=val1 --arg var2=val2
• add some syntatic sugar by separating with them , to avoir redundancy, good call

this would make it much more convenient to switch back and from vs pipe and preview-rs, (i'm the prolly the only one who use them that much)

[quietvoid]

There's an alias now.