[Feature] Dynamic Screenshots Path

[mousemke]

It seems fairly straightforward. There could be a screenshotPath option passed into imgfilepath. Is there a specific reason why this is not implemented? (dynamic or relative paths wouldnt work for some reason?) I would be willing to implement this into a PR as long as it's a desired thing

[mousemke]

also (for context, i'm coming to niffy from backstop) having the default path be local to a project instead of global ( /[whatever] )

[jessehattabaugh]

I too would like to have my screenshots in my project directory, because I want to put them in version control.