Fix empty stylesheets in build

Middleman compiles directly every file in `source/assets/stylesheets` which name doesn't start with `_`. That caused empty files in build:

```
⌘ ~/src/static-base/build/assets/stylesheets/ (master): ls -laR
total 200
drwxr-xr-x  5 vast  staff     170 Jun 21 23:17 .
drwxr-xr-x  5 vast  staff     170 Jun 21 23:14 ..
-rw-r--r--  1 vast  staff  101339 Jun 21 23:14 application-a016ed23.css
drwxr-xr-x  7 vast  staff     238 Jun 21 23:17 core
drwxr-xr-x  3 vast  staff     102 Jun 21 23:17 modules

./core:
total 0
drwxr-xr-x  7 vast  staff  238 Jun 21 23:17 .
drwxr-xr-x  5 vast  staff  170 Jun 21 23:17 ..
-rw-r--r--  1 vast  staff    0 Jun 21 23:17 base-da39a3ee.css
-rw-r--r--  1 vast  staff    0 Jun 21 23:17 content-da39a3ee.css
-rw-r--r--  1 vast  staff    0 Jun 21 23:17 helpers-da39a3ee.css
-rw-r--r--  1 vast  staff    0 Jun 21 23:17 layout-da39a3ee.css
-rw-r--r--  1 vast  staff    0 Jun 21 23:17 settings-da39a3ee.css

./modules:
total 0
drwxr-xr-x  3 vast  staff  102 Jun 21 23:17 .
drwxr-xr-x  5 vast  staff  170 Jun 21 23:17 ..
-rw-r--r--  1 vast  staff    0 Jun 21 23:17 example-da39a3ee.css
```

Adding `_` to filenames makes Middleman not ignore such files (so it generates only `application.css`):

```
⌘ ~/src/static-base/build/assets/stylesheets/ (fix-empty-stylesheets-in-build): ls -laR
total 200
drwxr-xr-x  3 vast  staff     102 Jun 21 23:20 .
drwxr-xr-x  5 vast  staff     170 Jun 21 23:14 ..
-rw-r--r--  1 vast  staff  101339 Jun 21 23:14 application-a016ed23.css
```