themes: Add heading-style for gruvbox and zt_blue themes, fixes zulip#1095

This commit along with ed3227b and 22a17fb implement heading-styles for all the themes.
h1 to h6 styles have the same implementation of 'msg_heading' in the respective themes as of now.

Author: mounilKshah

zulipterminal/themes/gruvbox.py

@@ -61,6 +61,7 @@ class GruvBoxColor(Enum):
     'msg_emoji'        : (Color.NEUTRAL_PURPLE,        Color.DARK0_HARD),
     'reaction'         : (Color.NEUTRAL_PURPLE__BOLD,  Color.DARK0_HARD),
     'reaction_mine'    : (Color.DARK0_HARD,            Color.NEUTRAL_PURPLE),
+    'msg_heading'      : (Color.DARK0_HARD__BOLD,      Color.BRIGHT_GREEN),
     'msg_math'         : (Color.DARK0_HARD,            Color.GRAY_244),
     'msg_mention'      : (Color.BRIGHT_RED__BOLD,      Color.DARK0_HARD),
     'msg_link'         : (Color.BRIGHT_BLUE,           Color.DARK0_HARD),

zulipterminal/themes/zt_blue.py

@@ -31,6 +31,7 @@
     'msg_emoji'       : (Color.DARK_MAGENTA,        Color.LIGHT_BLUE),
     'reaction'        : (Color.DARK_MAGENTA__BOLD,  Color.LIGHT_BLUE),
     'reaction_mine'   : (Color.LIGHT_BLUE,          Color.DARK_MAGENTA),
+    'msg_heading'     : (Color.WHITE__BOLD,         Color.BLACK),
     'msg_math'        : (Color.LIGHT_GRAY,          Color.DARK_GRAY),
     'msg_mention'     : (Color.LIGHT_RED__BOLD,     Color.LIGHT_BLUE),
     'msg_link'        : (Color.DARK_BLUE,           Color.LIGHT_GRAY),