Description of the desired feature

Currently, the user must use mapproject to determine the appropriate size for a inset to match a given region and projection. It would be useful to allow inset begin to accept the region and projection of the inset, similar to subplot begin, to avoid this extra step. Here is an example of the anticipated usage:

gmt begin test png
    gmt grdimage @earth_relief -R-51/-48/-24/-23 -JM15c -B -Cworld
    gmt inset begin -DjBL+o0.1c -F+p1p,black -R-80/-28/-43/10 -JM2c
        gmt coast -Wthin -Swhite -Ggray 
    gmt inset end
gmt end show

Are you willing to help implement and maintain this feature? Yes