Support setting up LFortran toolchain

[awvwgk]

Add support for setting up LFortran with the respective C compiler. The recommended setup for LFortran is currently via conda-forge.

[certik]

Conda-forge is the easiest, as it has prebuilt binaries for all platforms. I need to make a release and update the package though.

It can be built relatively easily as well on all platforms, but the easiest to get all dependencies is via Conda, so I guess one might as well just use LFortran via Conda anyway.

[wpbonelli]

#57, #79, #80 provide initial support for lfortran. But I wonder about the choice of C/++ compiler, and there are shell caveats on windows. this should probably stay open pending cleanup and until it sees some user testing.