Split into a main command package an an importable package.

[dchapes]

Should resolve #18, and also resolve #20.

Also, add go.mod and go.sum for Go module support.

While adjusting the README for the above changes, also expand on installation paths which should resolve #19.

[dchapes]

Note: I use mercurial with hggit so I'm not sure if the file copy of main.go to cmd/pxl/main.go was correctly detected as a repository file copy on the conversion to git (so that things like git blame within the new file will know what lines haven't been changed since the original).

Also note that this PR and my previous one at https://github.com/ichinaski/pxl/pull/21 are slightly incompatible in that they both muck with canvasSize in different ways, the former splits it out as unix and windows versions and this PR adds error handling. If you only want to accept one of these PRs and not the other for any reason, that'll work fine. If you do want both you can either adjust the error handing yourself or pick one to accept and I'll adjust/re-base the other to apply cleanly. Or, if you really want, I could combine the two PRs into a single new one.

Thanks.

[bieli]

@ichinaski It will be good to merge this PR. Everything was fine today when I cloned the related fork and ran the program with one command go build cmd/pxl/main.go. @dchapes +1