Read conf variables from the environment

[lrascao]

example conf:

param.p = $SOME_ENV_VARIABLE

cuttlefish would consult the environment for the SOME_ENV_VARIABLE key and replace it while processing the .conf file. I can put forward a PR for this, just wanted to first clear if this an appropriate syntax.

cc @lukebakken

[lrascao]

one other option is to use the current $() variable substitution and have it also search the environment.

[lukebakken]

My inclination is to use $()

[lukebakken]

Implemented in #17. Thank you!

[lrascao]

@lukebakken could you push 2.6.0 to hex please? thanks!

[lukebakken]

Please be patient! I was waiting on CI yesterday to complete when my day ended.

[lukebakken]

https://hex.pm/packages/cuttlefish/2.6.0

[lrascao]

thanks!