Add option variable for setting the main elm file

[trezona-lecomte]

This change adds an option-var for setting a main .elm file that will always be compiled when flycheck-elm checks any file.

This is helpful because the output of elm-make compilation is only usable if the .elm file has a 'main' function, so when flycheck-elm checks and compiles any non-main files, the resulting elm.js or index.html file isn't usable until the main elm file is compiled again.

[bsermons]

Sorry for the delay. Looks good though, thanks for the PR!

[trezona-lecomte]

No worries at all, thanks for the package.