Add nested shell level feature

Add display of the $SHLVL environment variable if it is above 1.

Resolves #606

docs/theme/included/powerline.rst

@@ -468,6 +468,12 @@ ______
 
    Color for the command runtime section.
 
+.. attribute:: POWERLINE_SHLVL_COLOR
+   :type: array<int>
+   :value: (231, 58, 0, 0, 7, 2)
+
+   Color for the nested shell level section.
+
 .. attribute:: POWERLINE_SOFTWARE_COLLECTIONS_COLOR
    :type: array<int>
    :value: (231, 62, 0, 0, 7, 5)

powerline.theme

@@ -183,6 +183,9 @@ _lp_powerline_full_theme_activate() {
     POWERLINE_RUNTIME_COLOR=( ${POWERLINE_RUNTIME_COLOR[@]+"${POWERLINE_RUNTIME_COLOR[@]}"} )
     [[ ${#POWERLINE_RUNTIME_COLOR[@]} == 0 ]] && POWERLINE_RUNTIME_COLOR=(226 17 0 0 3 4)
 
+    POWERLINE_SHLVL_COLOR=( ${POWERLINE_SHLVL_COLOR[@]+"${POWERLINE_SHLVL_COLOR[@]}"} )
+    [[ ${#POWERLINE_SHLVL_COLOR[@]} == 0 ]] && POWERLINE_SHLVL_COLOR=(231 58 0 0 7 2)
+
     POWERLINE_SOFTWARE_COLLECTIONS_COLOR=( ${POWERLINE_SOFTWARE_COLLECTIONS_COLOR[@]+"${POWERLINE_SOFTWARE_COLLECTIONS_COLOR[@]}"} )
     [[ ${#POWERLINE_SOFTWARE_COLLECTIONS_COLOR[@]} == 0 ]] && POWERLINE_SOFTWARE_COLLECTIONS_COLOR=(231 62 0 0 7 5)
 
@@ -250,6 +253,10 @@ _lp_powerline_full_theme_prompt() {
         __powerline_section "${LP_MARK_DIRSTACK}${lp_dirstack}" "${POWERLINE_DIRSTACK_COLOR[@]}"
     fi
 
+    if _lp_shell_level; then
+        __powerline_section "${LP_MARK_SHLVL}${lp_shell_level}" "${POWERLINE_SHLVL_COLOR[@]}"
+    fi
+
     local lp_chroot
     if _lp_chroot; then
         __powerline_section "${POWERLINE_CHROOT_MARKER}${lp_chroot}" "${POWERLINE_CHROOT_COLOR[@]}"