WTF-897 Exchange Rate improvements

Allows the user to set the precision for their exchange rate values.

Config setting:
```
exchangerates:
  precision: 3
```

Default is `7`.

Also aligns converted values along the decimal place for improved
aesthetics.

Signed-off-by: Chris Cummer <chriscummer@me.com>

wtf/numbers.go

@@ -0,0 +1,14 @@
+package wtf
+
+import "math"
+
+// Round rounds a float to an integer
+func Round(num float64) int {
+	return int(num + math.Copysign(0.5, num))
+}
+
+// TruncateFloat64 truncates the decimal places of a float64 to the specified precision
+func TruncateFloat64(num float64, precision int) float64 {
+	output := math.Pow(10, float64(precision))
+	return float64(Round(num*output)) / output
+}

wtf/numbers_test.go

@@ -0,0 +1,78 @@
+package wtf
+
+import (
+	"testing"
+
+	"gotest.tools/assert"
+)
+
+func Test_Round(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    float64
+		expected int
+	}{
+		{
+			name:     "negative",
+			input:    -3,
+			expected: -3,
+		},
+		{
+			name:     "integer",
+			input:    3,
+			expected: 3,
+		},
+		{
+			name:     "float down",
+			input:    3.123456,
+			expected: 3,
+		},
+		{
+			name:     "float up",
+			input:    3.998786,
+			expected: 4,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			actual := Round(tt.input)
+			assert.Equal(t, tt.expected, actual)
+		})
+	}
+}
+
+func Test_TruncateFloat(t *testing.T) {
+	tests := []struct {
+		name      string
+		input     float64
+		precision int
+		expected  float64
+	}{
+		{
+			name:      "negative precision",
+			input:     23.234567,
+			precision: -2,
+			expected:  0,
+		},
+		{
+			name:      "zero precision",
+			input:     23.234567,
+			precision: 0,
+			expected:  23,
+		},
+		{
+			name:      "positive precision",
+			input:     23.234567,
+			precision: 2,
+			expected:  23.23,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			actual := TruncateFloat64(tt.input, tt.precision)
+			assert.Equal(t, tt.expected, actual)
+		})
+	}
+}