Add capacity field to battery widget

[mphe]

This adds a capacityand n_capacity field to the battery widget. It contains the remaining battery capacity in percent. Useful to see the amount of capacity loss.

Unfortunately, I don't know how to update the wiki, so here is the diff:

diff --git a/bat.md b/bat.md
index fb98166..048c680 100644
--- a/bat.md
+++ b/bat.md
@@ -46,7 +46,9 @@ If you define `pspath`, **be sure** to not forget the final slash (/).
 - `perc`, total charge percentage (integer between 0 and 100 or "N/A");
 - `n_perc[i]`, i-th battery charge percentage (like above);
 - `time`, time remaining until charge if charging, until discharge if discharging (HH:MM string or "N/A");
-- `watt`, battery watts (float with 2 decimals).
+- `watt`, battery watts (float with 2 decimals);
+- `capacity`, remaining battery capacity in percent;
+- `n_capacity[i]`, i-th battery remaining capacity (like above).

 and can modify the following three tables, which will be the preset for the naughty notifications:
 * `bat_notification_charged_preset` (used if battery is fully charged and connected to AC)