Allow text files to keep customized parts when regenerating.

[mshima]

Overview of the feature request

Allow plain text files like gitignore, prettierignore, css files, others, to have a marker that identifies customization start.
Every time the marker is found, that part is extracted and appended to the end of the regenerated file.

Ex (.gitignore):

# Logs
logs
*.log

# JHipster keep
MyFile
MyFolder

Motivation for or Use Case

Make easier to keep plain text files customizations between regenerations.

Related issues or PR

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