Add support for steam:// and .lnk

[Loxtir]

Aims to implement #21, I totally get it if you reject it tho haha.
The whole process is a pain in the ass.

First, the only way I found to get the PID of the process started with cmd is through executing tasklist and filtering for its name. Problem? When you launch a .lnk or use the steam protocol, the name that appears on tasklist is the .exe name. So I had to add "exe_name" as a new field in the config file.

Second, what if you have another instance of that process running before? So I had to check before creating the new process, store the previous PIDs and compare after.

Third, launching with steam:// takes a while, sometimes up to 15s in my computer. This means the new process isn't created until that, so I had to delay the PID search, that's why there's retries in on_timer_timeout().
Last, there's no way I found to get the exit code of a process spawned this way, so relaunching on crash is impossible unless we only allow closing the app through the launcher.

I tested it with a steam game, a .lnk and a regular game and everything seems to work OK.

PD: in theory, execute_with_pipe() gives you access to the stdio of the cmd it opens, but I wasn't able to read or write anything from/to it. Since it's non blocking, my idea was to wait the process by passing /wait, and then echo the %ERRORLEVEL% since it that gives the exit code of the previous process spawned by a cmd. Ideas/contributions are welcome.