Allow displaying co_firstlineno in -F mode

[Jongy]

In --function mode, py-spy doesn't write line numbers (to avoid aggregating by the "last instruction" line numbers - and instead, by function name).
Function name is not always enough to identify a function in a file - for example, __init__ in a file with multiple classes.
I suggest adding py-spy the ability to to log the co_firstlineno when in --function mode (or at least, make it an option with an extra flag).

The business logic is simple:

 /// Gets a stack trace for an individual thread
-pub fn get_stack_trace<T>(thread: &T, process: &Process, copy_locals: bool) -> Result<StackTrace, Error>
+pub fn get_stack_trace<T>(thread: &T, process: &Process, copy_locals: bool, line_no: LineNo) -> Result<StackTrace, Error>
         where T: ThreadState {
     // TODO: just return frames here? everything else probably should be returned out of scope
     let mut frames = Vec::new();
@@ -93,15 +99,19 @@ pub fn get_stack_trace<T>(thread: &T, process: &Process, copy_locals: bool) -> R
         let filename = copy_string(code.filename(), process).context("Failed to copy filename")?;
         let name = copy_string(code.name(), process).context("Failed to copy function name")?;
 
-        let line = match get_line_number(&code, frame.lasti(), process) {
-            Ok(line) => line,
-            Err(e) => {
-                // Failling to get the line number really shouldn't be fatal here, but
-                // can happen in extreme cases (https://github.com/benfred/py-spy/issues/164)
-                // Rather than fail set the linenumber to 0. This is used by the native extensions
-                // to indicate that we can't load a line number and it should be handled gracefully
-                warn!("Failed to get line number from {}.{}: {}", filename, name, e);
-                0
+        let line = match line_no {
+            NoLine => 0,
+            FirstLineNo => code.first_lineno(),
+            LastInstruction => match get_line_number(&code, frame.lasti(), process) {
+                Ok(line) => line,
+                Err(e) => {
+                    // Failling to get the line number really shouldn't be fatal here, but
+                    // can happen in extreme cases (https://github.com/benfred/py-spy/issues/164)
+                    // Rather than fail set the linenumber to 0. This is used by the native extensions
+                    // to indicate that we can't load a line number and it should be handled gracefully
+                    warn!("Failed to get line number from {}.{}: {}", filename, name, e);
+                    0
+                }
             }

I'm wondering how to add it with respect to py-spy existing flags (config.show_line_numbers). I can change show_line_number to LineNo and pass it over in py-spy instead? Or add it as an extra option?

Wdyt @benfred ?