Fix PropertyFormatLinter to handle interpolation

Previously, the `PropertyFormatLinter` would incorrectly report lints
for situations such as:

    margin-#{$side}: $value;

This was due to the fact that the regex was hard-coded to assume there
were only alphabetical characters and hyphens in the property name.

To fix this, we adjust the regex to support ignoring interpolation
characters.

Fixes #5 and #15.

Change-Id: Ifbdb73a955b8a9e687f129f10b3c1fa7d42d7490
Reviewed-on: https://gerrit.causes.com/23140
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>