Redundant `imagetype` argument

[stefaneng]

If filename is specified with an extension then imagetype parameter should be inferred automatically.

Two solutions:

1. Leaving both arguments in I propose that we:
   a. First look for extension in filename
   b. If no extension then find extension from imagetype
2. Deprecate imagetype argument and only access filetype via extension given in filename

Current we can create a tiff file with a pdf extension without any kind of warning:

venn.diagram(
	x = list(A = 1:150, B = 121:170),
	filename = 'test.pdf',
	imagetype = 'tiff',
	disable.logging = TRUE
	)

[pboutros]

Yup makes sense to deprecate imagetype, and potentially to import the write.plot function that's in BPG