add email templater with basic if statements

at this point I really should've just used text/template, but I guess
this way compatibility is kept with existing custom emails. If statement
works as so:

{if variable}variable was true{endif}
{if !variable}variable was false{endif}

no else yet, just do as above (two if statements).

template.go

@@ -0,0 +1,118 @@
+package main
+
+import "fmt"
+
+func truthy(val interface{}) bool {
+	switch v := val.(type) {
+	case string:
+		return v != ""
+	case bool:
+		return v
+	case int:
+		return v != 0
+	}
+	return false
+}
+
+// Templater for custom emails.
+// Variables should be written as {varName}.
+// If statements should be written as {if (!)varName}...{endif}.
+// Strings are true if != "", ints are true if != 0.
+func templateEmail(content string, variables []string, conditionals []string, values map[string]interface{}) string {
+	ifStart, ifEnd := -1, -1
+	ifTrue := false
+	invalidIf := false
+	previousEnd := -2
+	cStart, cEnd := -1, -1
+	varStart, varEnd := -1, -1
+	varName := ""
+	out := ""
+	for i, c := range content {
+		if c == '{' {
+			cStart = i + 1
+			for content[cStart] == ' ' {
+				cStart++
+			}
+			if content[cStart:cStart+3] == "if " {
+				varStart = cStart + 3
+				for content[varStart] == ' ' {
+					varStart++
+				}
+			}
+			if ifStart == -1 {
+				out += content[previousEnd+2 : i]
+			}
+			if content[cStart:cStart+5] != "endif" || invalidIf {
+				continue
+			}
+			ifEnd = i - 1
+			if ifTrue {
+				b := templateEmail(content[ifStart:ifEnd+1], variables, conditionals, values)
+				out += b
+				ifTrue = false
+			}
+		} else if c == '}' {
+			if varStart != -1 {
+				ifStart = i + 1
+				varEnd = i - 1
+				for content[varEnd] == ' ' {
+					varEnd--
+				}
+				varName = content[varStart : varEnd+1]
+				positive := true
+				if varName[0] == '!' {
+					positive = false
+					varName = varName[1:]
+				}
+				validVar := false
+				wrappedVarName := "{" + varName + "}"
+				for _, v := range conditionals {
+					if v == wrappedVarName {
+						validVar = true
+						break
+					}
+				}
+				if validVar {
+					ifTrue = positive == truthy(values[varName])
+				} else {
+					invalidIf = true
+					ifStart, ifEnd = -1, -1
+				}
+				varStart, varEnd = -1, -1
+			}
+			cEnd = i - 1
+			for content[cEnd] == ' ' {
+				cEnd--
+			}
+			previousEnd = i - 1
+			if content[cEnd-4:cEnd+1] == "endif" && !invalidIf {
+				continue
+			}
+			validVar := false
+			varName = content[cStart : cEnd+1]
+			cStart, cEnd = -1, -1
+			if ifStart != -1 {
+				continue
+			}
+			wrappedVarName := "{" + varName + "}"
+			for _, v := range variables {
+				if v == wrappedVarName {
+					validVar = true
+					break
+				}
+			}
+			if !validVar {
+				out += wrappedVarName
+				continue
+			}
+			out += fmt.Sprint(values[varName])
+		}
+	}
+	if previousEnd+1 != len(content)-1 {
+		out += content[previousEnd+2:]
+	}
+	if out == "" {
+		return content
+	}
+	return out
+}