cmd/abigen: Add support for STDIN to abigen

Allow the --abi flag to be given - to indicate that it should read the
ABI information from standard input. It expects to read the solc output
with the --combined-json flag providing bin, abi, userdoc, devdoc, and
metadata, and works very similarly to the internal invocation of solc,
except it allows external invocation of solc.

This facilitiates integration with more complex solc invocations, such
as invocations that require path remapping or --allow-paths tweaks.

Simple usage example:

    solc --combined-json bin,abi,userdoc,devdoc,metadata *.sol | abigen --abi -

cmd/abigen/main.go

@@ -29,7 +29,7 @@ import (
 )
 
 var (
-	abiFlag = flag.String("abi", "", "Path to the Ethereum contract ABI json to bind")
+	abiFlag = flag.String("abi", "", "Path to the Ethereum contract ABI json to bind, - for STDIN")
 	binFlag = flag.String("bin", "", "Path to the Ethereum contract bytecode (generate deploy method)")
 	typFlag = flag.String("type", "", "Struct name for the binding (default = package name)")
 
@@ -75,16 +75,27 @@ func main() {
 		bins  []string
 		types []string
 	)
-	if *solFlag != "" {
+	if *solFlag != "" || *abiFlag == "-" {
 		// Generate the list of types to exclude from binding
 		exclude := make(map[string]bool)
 		for _, kind := range strings.Split(*excFlag, ",") {
 			exclude[strings.ToLower(kind)] = true
 		}
-		contracts, err := compiler.CompileSolidity(*solcFlag, *solFlag)
-		if err != nil {
-			fmt.Printf("Failed to build Solidity contract: %v\n", err)
-			os.Exit(-1)
+
+		var contracts map[string]*compiler.Contract
+		var err error
+		if *solFlag != "" {
+			contracts, err = compiler.CompileSolidity(*solcFlag, *solFlag)
+			if err != nil {
+				fmt.Printf("Failed to build Solidity contract: %v\n", err)
+				os.Exit(-1)
+			}
+		} else {
+			contracts, err = contractsFromStdin()
+			if err != nil {
+				fmt.Printf("Failed to read input ABIs from STDIN: %v\n", err)
+				os.Exit(-1)
+			}
 		}
 		// Gather all non-excluded contract for binding
 		for name, contract := range contracts {
@@ -138,3 +149,12 @@ func main() {
 		os.Exit(-1)
 	}
 }
+
+func contractsFromStdin() (map[string]*compiler.Contract, error) {
+	bytes, err := ioutil.ReadAll(os.Stdin)
+	if err != nil {
+		return nil, err
+	}
+
+	return compiler.ParseCombinedJSON(bytes, "", "", "", "")
+}