Add untildify

Converts a tilde path to an absolute path.

snippets/untildify.md

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+### untildify
+
+Converts a tilde path to an absolute path.
+
+Use `String.replace()` with a regular expression and `OS.homedir()` to replace the `~` in the start of the path with the home directory.
+
+```js
+const untildify = str => str.replace(/^~($|\/|\\)/, `${require('os').homedir()}$1`);
+```
+
+```js
+untildify('~/node') // '/Users/aUser/node'
+```