Fix linter not work on Windows/MinGW-x64

[machsix]

In previous version, to make linter works on Windows, there are the following three requirements:

1. Install MinGW-x64 version gfortran compiler. NOT MSYS2, NOT Cygwin. Only MinGW-x64 can run natively on windows. You can install it by first install mysys2 and then run pacman -Syu and pacman -S mingw-w64-x86_64-toolchain.
2. Add the bin directory to the PATH environment variable, for my case, it's C:\msys64\mingw64\bin
3. Add the path of gfortran to the Gfortran Excutable, for my case, it's C:\msys64\mingw64\bin\gfortran.exe

This PR removes the second requirement since editing PATH globally can ruin other program. The fix extracts the path of bin directory of gfortran and adds it to the PATH of child_process

Fix #116

[pedro-ricardo]

Looks good to me!

[machsix]

travis shows the following error because linux has env.PATH instead of env.Path. I tried to add // @ts-ignore ahead of the lines but prettier seems to strip that out.

> tsc -p ./
src/features/linter-provider.ts:42:16 - error TS2339: Property 'Path' does not exist on type '{ LC_ALL: string; }'.
42       if (!env.Path.includes(path.dirname(command))) {
                  ~~~~
src/features/linter-provider.ts:43:13 - error TS2339: Property 'Path' does not exist on type '{ LC_ALL: string; }'.
43         env.Path = `${path.dirname(command)}${path.delimiter}${env.Path}`;
               ~~~~
src/features/linter-provider.ts:43:68 - error TS2339: Property 'Path' does not exist on type '{ LC_ALL: string; }'.
43         env.Path = `${path.dirname(command)}${path.delimiter}${env.Path}`;
                                                                      ~~~~