Improved bot reaction to messages to correctly recognize hello/help words

Related to Issue #5; improved hello/help function with following new features:

- correctly recognizes messages containing hello/help requests, only if they are separate words in the message (doesn't recognize things like "xxhixx WelcomeBot", recognizes "Hi, WelcomeBot!")
- accepts various forms of these hello/help words:
  - uppercase: accepts uppercase words or combinations of upper and lowercase letters constituting the word (e.g. "Hey WelcomeBot", "HEY WelcomeBot", "heY WelcomeBot")
 - supports word lengthening: recognizes hello/help words that contain repeated letters for emphasis (e.g. "Heyyy WelcomeBot", "WelcomeBot helppp")

To do:
- currently it doesn't support multi-word hello/help words like "explain yourself" - these will not be recognized by the bot.