Use only basename when forming stem from script and tag

[kylebaron]

See example in #52

There's a feature where you can set a context and save a series of plots using a combination of the context name and a "tag" that is supplied with each plot.

> mrggsave:::make_stem("foo.R", tag = "plot1")
[1] "foo-plot1"

The default "context" is the name of the script, but you can override that through an option.

This was causing a problem when we started passing in the full path to the script:

> mrggsave:::make_stem("script/foo.R", tag = "plot1")
[1] "script/foo-plot1"

This PR just takes the basename() of script prior to starting this whole process off to ensure we don't get a path to output file that we didn't intend.

The change here is only meant to preserve existing behavior, which was expecting script to be just the name of the script.

[kylebaron]

@KatherineKayMRG heads up.