Add command and variable substitution examples.

Author: nsaunders

examples/CommandSubstitution.purs

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+module Example.CommandSubstitution (main) where
+
+import Prelude
+
+import Data.Maybe (maybe)
+import Dotenv (loadContents) as Dotenv
+import Effect (Effect)
+import Effect.Aff (launchAff_, throwError)
+import Effect.Class (liftEffect)
+import Effect.Console (log)
+import Effect.Exception (error)
+import Node.Process (lookupEnv)
+
+main :: Effect Unit
+main = launchAff_ do
+  Dotenv.loadContents "GREETING=Hello, $(whoami)!" -- Normally you'll use `loadFile` instead.
+  liftEffect do
+    greeting <- lookupEnv "GREETING"
+    maybe (throwError $ error $ "Couldn't find GREETING!") log greeting

examples/VariableSubstitution.purs

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+module Example.VariableSubstitution (main) where
+
+import Prelude
+
+import Data.Maybe (maybe)
+import Dotenv (loadContents) as Dotenv
+import Effect (Effect)
+import Effect.Aff (launchAff_, throwError)
+import Effect.Class (liftEffect)
+import Effect.Console (log)
+import Effect.Exception (error)
+import Node.Process (lookupEnv)
+
+main :: Effect Unit
+main = launchAff_ do
+  Dotenv.loadContents "WHO=World\nGREETING=Hello, ${WHO}!" -- Normally you'll use `loadFile` instead.
+  liftEffect do
+    greeting <- lookupEnv "GREETING"
+    maybe (throwError $ error $ "Couldn't find GREETING!") log greeting