feat: add ability to execute custom scripts and commands

[Ukalnins]

This is a resubmit of the #338 with a different approach.

This does not require the custom script to re implement the whole fruitstrap python script, but, if required, override some functionality by specifying own implementation of the function.

[gabebear]

Thanks, this does solve my concerns from #338. Does this mean any scripts used with this option need to be pre-processed to add the escaping that the Xcode project uses awk to add? E.g.
shellScript = "echo \"\\\"# AUTO-GENERATED - DO NOT MODIFY\\\\\\n\\\"\" > src/ios-deploy/lldb_cmds.h\nawk '{ print \"\\\"\"$0\"\\\\n\\\"\"}' src/scripts/lldb_cmds >> src/ios-deploy/lldb_cmds.h\n";

[Ukalnins]

No. The escaping is needed for stuff that is embedded in c source but not custom scripts.

The xcode stuff is used to transform the py file

abc = "cde"

x = 1

to the c string like:

char * contents = "abc = \"cde\"\n\nx = 1";

As we are not including the custom scripts into c code, no preprocessing is required.

[gabebear]

Cool, this will make things easier for many people