Add at-import-partial-extension rule

src/rules/at-import-partial-extension/README.md

@@ -0,0 +1,116 @@
+# at-import-partial-extension
+
+Require or disallow extension in `@import` commands.
+
+```scss
+@import "file.scss";
+/**           ↑
+ * This extension */
+```
+
+The rule ignores [cases](http://sass-lang.com/documentation/file.SASS_REFERENCE.html#import) when Sass considers an `@import` command just a plain CSS import:
+
+- If the file’s extension is `.css`.
+- If the filename begins with `http://` (or any other protocol).
+- If the filename is a `url()`.
+- If the `@import` has any media queries.
+
+## Options
+
+`string`: `"always"|"never"`
+
+### `"always"`
+
+The following patterns are considered warnings:
+
+```scss
+@import "foo";
+```
+
+```scss
+@import "path/fff";
+```
+
+```scss
+@import "path\\fff";
+```
+
+```scss
+@import "df/fff", "1.SCSS";
+```
+
+The following patterns are _not_ considered warnings:
+
+```scss
+@import "fff.scss";
+```
+
+```scss
+@import "path/fff.scss";
+```
+
+```scss
+@import url("path/_file.css"); /* has url(), so doesn't count as a partial @import */
+```
+
+```scss
+@import "file.css"; /* Has ".css" extension, so doesn't count as a partial @import */
+```
+
+```scss
+/* Both are URIs, so don't count as partial @imports */
+@import "http://_file.scss";
+@import "//_file.scss";
+```
+
+```scss
+@import "file.scss" screen; /* Has a media query, so doesn't count as a partial @import */
+```
+
+### `"never"`
+
+The following patterns are considered warnings:
+
+```scss
+@import "foo.scss";
+```
+
+```scss
+@import "path/fff.less";
+```
+
+```scss
+@import "path\\fff.ruthless";
+```
+
+```scss
+@import "df/fff", "1.SCSS";
+```
+
+The following patterns are _not_ considered warnings:
+
+```scss
+@import "foo";
+```
+
+```scss
+@import "path/fff";
+```
+
+```scss
+@import url("path/_file.css"); /* has url(), so doesn't count as a partial @import */
+```
+
+```scss
+@import "file.css"; /* Has ".css" extension, so doesn't count as a partial @import */
+```
+
+```scss
+/* Both are URIs, so don't count as partial @imports */
+@import "http://_file.scss";
+@import "//_file.scss";
+```
+
+```scss
+@import "file.scss" screen; /* Has a media query, so doesn't count as a partial @import */
+```