nmslib.CreateBanner() now calls nmslib.GenRawBanner() for the
majority of processing createbanner now just calls the gen func
and saves the file.
GenRawBanner() returns an image.Image type for more processing by
another function. this will be used for upcoming features.
tests have been updated as needed
upddated go.mod and go.sum
**resource.go/assets.go**
* renamed `resources.go` to `assets.go`
* added dark glyphs - located in assets.go -> assets/glyphs/dark/
* moved original glyphs - located in assets.go -> assets/glyph/light/
**glyphbanner.go**
* added option to make glyphs vertical and/or dark
**rpcg.go**
* Added a random portal code generator this function takes no args and returns a string
**README.md**
* Fixed some typos (probably more to be fixed)
* Added example of vertical portal banner
* Added list of options for banner creation
* Added example of ``nmslib.RndPortal()`` usage
**go.mod**
* updated go version to 1.16
