Fix #6661. Configurable multiple linters.
Add testcase for non-existance provider.

Test that non-existing provider, preferredOnly = true and firstOnly =
true.
Add test case for when prefer is not specified.

Also, change single-quote to double-quotes for string literals.
Update strings.js

Minor changes were made to the following sections in order to express the sentence or phrase more fluently for the reader in the native language.
- "ERROR_PREFS_CORRUPT"
- "ERROR_IN_BROWSER" 
- "ERROR_MAX_FILES"
- "ERROR_LAUNCHING_BROWSER_TITLE"
- "ERROR_FETCHING_UPDATE_INFO_MSG"
- "ERROR_QUICK_EDIT_PROVIDER_NOT_FOUND"
- "ERROR_QUICK_DOCS_PROVIDER_NOT_FOUND"
- "ABOUT_TEXT_WEB_PLATFORM_DOCS"
- "CANNOT_WRITE_TEMP"
Update strings.js

Minor changes were made to the following sections in order to express the sentence or phrase more fluently for the reader in the native language.
Update strings.js

Fixed 2 spaces between con and proyectos as per TomMalbran revision.
Update strings.js
Update strings.js

Revert SHA as per @TomMalbran
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/mackenza/brackets
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of https://github.com/mackenza/brackets
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'