feat: support build step

[axetroy]

[package]
name = "github.com/axetroy/gpm.rs"
bin = "gpm"
versions = ["0.1.12", "0.1.11"]
authors = ["Axetroy <axetroy.dev@gmail.com>"]
keywords = ["gpm", "git", "project", "manager"]
repository = "https://github.com/axetroy/gpm.rs"
description = """
A command line tool, manage your hundreds of repository, written with Rust.
"""

[build.unix]
sh = """
git clone https://github.com/axetroy/gpm.rs
make
"""
[[build.unix.binary]]
name = "gpm"
path = "./gpm"

[build.windows]
cmd = """
git clone https://github.com/axetroy/gpm.rs
make
"""
[[build.windows.binary]]
name = "gpm"
path = "./gpm.exe"

This formula will clone https://github.com/axetroy/gpm.rs and then run make command.

[iawia002]

Looks good, I'm wondering that since we've already configured the git repository address in the package section, we could run git clone first by default, and just write the build command in sh/cmd.

Another small question is why the parameter name is sh on Unix and cmd on Windows, they mean the same thing, can they be unified?