Support empty lines and comments.

In debug mode, show errors like:
```
[dotenv][DEBUG] did not match key and value when parsing line <n>:
[dotenv][DEBUG] did not match key and value when parsing line <n>: #<COMMENTED_LINE>
```
Sometimes we need to add empty lines to separate groups of variables, or comment some of them.
We don't want the debugger or the parser catch them.

lib/main.js

@@ -39,6 +39,11 @@ function parse (src /*: string | Buffer */, options /*: ?DotenvParseOptions */)
 
   // convert Buffers before splitting into lines and processing
   src.toString().split(NEWLINE).forEach(function (line, idx) {
+    line = line.trim()
+    // ignore empty and commented lines
+    if (line === '' || line[0] === '#') {
+      return
+    }
     // matching "KEY' and 'VAL' in 'KEY=VAL'
     const keyValueArr = line.match(RE_INI_KEY_VAL)
     // matched?