Task for pre-processing source files

I'd like to contribute to MSBuild a task we wrote for aspnet that I think would be generally useful. Is this something MSBuild would accept as a PR?

### GenerateFileFromTemplate

Purpose: pre-process a source file using MSBuild variables.
**Usage:**
```xml

```
**Template file example**
```
// MyFile.cs.in
public class ${ClassName}
{
public void Method${MethodName}()
{
Console.WriteLine("`${NotReplacedDueToBacktick}"");
}
}
```

**Template syntax**:
* variables in the template file use powershell/bash syntax: `${VarName}`.
* if you need a file with `${...}`, you can escape this with a backtick: `${...}

**Justification**:
WriteLinesToFile is good, but I've frequently run into problems with it when I need to write a file that has `\`, `/`, or `;` in it.

Current implementation: https://github.com/aspnet/BuildTools/blob/master/modules/BuildTools.Tasks/GenerateFileFromTemplate.cs, https://github.com/aspnet/BuildTools/blob/master/test/BuildTools.Tasks.Tests/GenerateFileFromTemplateTests.cs

cc @jeffkl @livarcocc