Field to specify minimum version of the Nirum compiler

[dahlia]

A Nirum package may use recently introduced features of the compiler so that it requires the minimum version of the compiler. Although it could be achieved by simply adding a new field to package.toml manifest:

version = "1.0.0"
compiler_requires = ">= 0.3.0"

The syntax also could be unified to #26:

version = "1.0.0"

[dependencies]
"http://nirum.org/" = ">= 0.3.0"