Inherit existing queries

[privatenumber]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
I am working on a loader that compiles markdown into a Vue component and also extracts all Vue code-blocks into demos. I do this by creating a mddemo-loader that extracts the particular demo by an index passed into the query (eg. idx=0). The output of this loader is piped into vue-loader. However, vue-loader currently doesn't inherit the existing queries so the mddemo-loader doesn't get applied (via webpack config) and the entire markdown is gets parsed when vue-loader tries to extract the template and script tags of a particular demo in the markdown file.

[yyx990803]

Can you add a test case for this?

[privatenumber]

@yyx990803 Added!