Added process environment provider

Show environment variables defined for a process with the given process
ID:

```bash
checkenv proc+<pid>
```

Author: zomglings

process_linux.go

@@ -0,0 +1,79 @@
+// Gets process environment by PID in Linux operating systems.
+
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func dropNullByte(data []byte) []byte {
+	size := len(data)
+	if size > 0 && data[size-1] == '\000' {
+		return data[0 : size-1]
+	}
+	return data
+}
+
+func SplitProcEnviron(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF {
+		if len(data) == 0 {
+			return 0, nil, nil
+		} else {
+			return len(data), dropNullByte(data), nil
+		}
+	}
+
+	if data[0] == '\000' {
+		return 1, nil, nil
+	}
+
+	if i := bytes.IndexByte(data, '\000'); i > 0 {
+		return i + 1, dropNullByte(data[0:i]), nil
+	}
+
+	return 0, nil, nil
+}
+
+func ProcessEnvProvider(filter string) (map[string]string, error) {
+	vars := map[string]string{}
+
+	// TODO(zomglings): For now, we expect the filter to contain the process PID but we will later
+	// want to add other things, like whitelists or blacklists of environment variables to read.
+	pid, pidErr := strconv.ParseUint(filter, 10, 64)
+	if pidErr != nil {
+		return vars, pidErr
+	}
+	filename := fmt.Sprintf("/proc/%d/environ", pid)
+
+	ifp, fileErr := os.Open(filename)
+	if fileErr != nil {
+		return vars, fileErr
+	}
+	defer ifp.Close()
+
+	scanner := bufio.NewScanner(ifp)
+	scanner.Split(SplitProcEnviron)
+	for scanner.Scan() {
+		line := scanner.Text()
+		components := strings.Split(line, "=")
+		name := components[0]
+		var value string
+		if len(components) > 1 {
+			value = strings.Join(components[1:], "=")
+		}
+		vars[name] = value
+	}
+
+	scanErr := scanner.Err()
+	return vars, scanErr
+}
+
+func init() {
+	helpString := "Provides the environment variables set for the process with the given pid."
+	RegisterPlugin("proc", helpString, noop, ProcessEnvProvider)
+}