Migrate from Dep to Go Modules

Go Modules are the preferred way to manage dependencies in Go programs
since version 1.11. The migration process involves:

 1. go mod init git.dominic-ricottone.com/~dricottone/image2ascii
    (replace the host as needed)
 2. Replace the host in files requiring an internal module
    (this was specifically image2ascii.go, convert/convert.go,
    convert/resize.go, and convert/resize_test.go)
 3. go mod tidy
 4. rm -rf vendor/ Gopkg.lock Gopkg.toml
 5. go get -u

go.mod now contains all the information required to build the program.

remove useless code

Refactor the code to match the oop mode for easy mock and test

add the fit options to scale the image to fit the screen

add reverse options

add main test case

add colored the asicc output

fixed the issue can the filename is null

Parse the cli args