build: -pkg flag to specify package name (#150)

Allows override in the case where package name is incorrectly deduced from directory name.

Fixes #68
Fixes #147

tests/fixedbugs/issue68/asm.go

@@ -0,0 +1,18 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Issue68", NOSPLIT, "func() uint64")
+	Doc("Issue68 tests custom package names.")
+	x := GP64()
+	MOVQ(U32(68), x)
+	Store(x, ReturnIndex(0))
+	RET()
+	Generate()
+}