Support cshtml files

[D-Bullock]

Hey, great work developing this.
We are using angular in conjunction with ASP.NET Razor pages. That means that we have essentially straight HTML files with the extension .cshtml.

It would be great if the extractor could parse them: it currently ignores them unless I rename them to .html (then it works perfectly).

[rubenv]

Relates to rubenv/angular-gettext-tools#22 (closing this one as it's a duplicate).

It's a one-liner fix, but I'd like to have a sample file + test case to make sure it keeps working.