Allow lupdate to affect only specified contexts

I'm looking for a way to update only specific contexts of .ts files via lupdate.exe. 

We have a large project with one context for each source file and running lupdate over the whole project takes some time. 
If a translation key is added in a single file (single context) it would be great if lupdate was able to update only that context without parsing the whole project.

Sugggestion

Add a command line parameter to lupdate.exe that allows to restict the process to the specified context OR restrict the process to the context of the specified file.

Alternative: Add a command line parameter to lupdate.exe that prevents the process from setting existing translations to "vanished".

Current Issue

If lupdate is called with a single file argument, all existing translations of other files (other contexts) in the target .ts file will be set to "vanished".