Add wireless signal strength data source

Has backends for Linux and MacOS. Displays the signal strength if it
falls below the configured threshold.

Includes a new method to test getting data directly from files, so this
is fully unit tested.

Resolves #139

docs/theme/included/powerline.rst

@@ -544,3 +544,11 @@ ______
    :value: (33, 17, 0, 0, 5, 4)
 
    Color for the current time section.
+
+.. attribute:: POWERLINE_WIFI_STRENGTH_COLOR
+   :type: array<int>
+   :value: (-1, 148, 0, 0, -1, 3)
+
+   Color for the wireless signal strength section.
+
+   .. versionadded:: 2.1

powerline.theme

@@ -204,6 +204,9 @@ _lp_powerline_full_theme_activate() {
 
     POWERLINE_TIME_COLOR=( ${POWERLINE_TIME_COLOR[@]+"${POWERLINE_TIME_COLOR[@]}"} )
     [[ ${#POWERLINE_TIME_COLOR[@]} == 0 ]] && POWERLINE_TIME_COLOR=(33 17 0 0 5 4)
+
+    POWERLINE_WIFI_STRENGTH_COLOR=( ${POWERLINE_WIFI_STRENGTH_COLOR[@]+"${POWERLINE_WIFI_STRENGTH_COLOR[@]}"} )
+    [[ ${#POWERLINE_WIFI_STRENGTH_COLOR[@]} == 0 ]] && POWERLINE_WIFI_STRENGTH_COLOR=(-1 148 0 0 -1 3)
 }
 
 _lp_powerline_full_theme_directory() {
@@ -234,6 +237,11 @@ _lp_powerline_full_theme_prompt() {
         __powerline_formatted_section "$lp_temperature_color" "${POWERLINE_TEMPERATURE_COLOR[@]}"
     fi
 
+    local lp_wifi_signal_strength_color
+    if _lp_wifi_signal_strength_color; then
+        __powerline_formatted_section "$lp_wifi_signal_strength_color" "${POWERLINE_WIFI_STRENGTH_COLOR[@]}"
+    fi
+
     local lp_detached_sessions lp_running_jobs lp_stopped_jobs jobs_string=
     if _lp_detached_sessions; then
         jobs_string="${lp_detached_sessions}d"