Feature request: Listing remote versions

[umireon]

Related to #22

This PR adds listing remote Lua, LuaJIT, and Luarocks versions.

$ luaver list -r
5.3.4
5.3.3
...
5.0.1
5.0

$ luaver list-luajit -r
2.0.4
2.1.0-beta2
...
1.1.0
1.0.3

$ luaver list-luarocks -r
2.4.2
2.4.1
...
0.3.2
0.3.1

Candidate versions are pulled from following pages by using awk and sed.

• Lua: https://www.lua.org/ftp/
• LuaJIT: http://luajit.org/download.html
• Luarocks: http://luarocks.github.io/luarocks/releases/releases.json

Note that candidate versions can be pulled from git tags but they have an imcomplete list of versions.

[umireon]

Added a commit to sort versions according to semver conventions.

[DhavalKapil]

This prints version in the reverse order on my machine:

$ luaver list -r
5.0
5.0.1
5.0.2
5.0.3
5.1
5.1.1
5.1.2
5.1.3
5.1.4
5.1.5
5.2.0
5.2.1
5.2.2
5.2.3
5.2.4
5.3.0
5.3.1
5.3.2
5.3.3
5.3.4

Anyways, I'm merging it for now. Thanks!