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>

modules/exchangerates/settings.go

@@ -15,6 +15,8 @@ const (
 type Settings struct {
 	common *cfg.Common
 
+	precision int `help:"How many decimal places to display." optional:"true"`
+
 	rates map[string][]string `help:"Defines what currency rates we want to know about"`
 	order []string
 }
@@ -24,6 +26,8 @@ func NewSettingsFromYAML(name string, ymlConfig *config.Config, globalConfig *co
 	settings := Settings{
 		common: cfg.NewCommonSettingsFromModule(name, defaultTitle, defaultFocusable, ymlConfig, globalConfig),
 
+		precision: ymlConfig.UInt("precision", 7),
+
 		rates: map[string][]string{},
 		order: []string{},
 	}