added \!optional tag to @extends that were throwing errors on compile.

When compiling, the following errors were thrown:

error sass/style.scss (Line 9 of sass/base/_grid-extends.scss: "#primary" failed to @extend ".content-area".
The selector ".content-area" was not found.
Use "@extend .content-area !optional" if the extend should be able to fail.
)

AND

error sass/style.scss (Line 40 of sass/base/_grid-extends.scss: ".attachment #primary" failed to @extend ".image-attachment".
The selector ".image-attachment" was not found.
Use "@extend .image-attachment !optional" if the extend should be able to fail.
)

added !optional tag to @extends to prevent those errors.

also added an additional ignore filetype.