Fixes #50

* [feat] THEMES.md: Add screenshots

What?:
- Add screens for all themes
- Add THEMES.md to showcase all screens

Breaking Changes? None

* fix #114

What?:
- Add screenshots to be added to wiki

Breaking Changes? None

* oh-my-bash.sh : Fix for #70

* Future proofing

* Fixed symlink for agnoster-bash.theme.sh

* Reaction on https://github.com/ohmybash/oh-my-bash/pull/71#discussion_r307617303

Accept just a bookmark name as a short form of the '-g' option.  If just
    one arg is supplied, and it doesn't start with a dash, then it is assumed
    to be a bookmark name and _goto_bookmark is called. [ bm BOOKMARK_NAME ]
  Add a -h option to print usage info with same behavior as supplying no args
  Add echoing an error message if an unrecognized option is given, followed
    by usage info
  Augment usage info to reflect these changes and update README.md

* README: Sanitization for local repos

* img: Added new directory for images

* fixed syntax err

* OMB - Major Refactor

- Aliases and completions now works like plugins (need to enabled in .bashrc)
- Removed the compatible check in spectrum.sh, OMB now works with Bash v3.x like the old days.
- Removed core plugin, added those bash functions into base.sh and load during startup.
- Updated OSH template for new installations
- Added history config and few other stuff from #17

@TODO: Added a shell script to update old version of .bashrc to new one.

* Fixed ShellCheck issues

* Fixed ShellCheck issues

set "function" before append_prompt_override in line 521 so that bash could known the comman

* Fix to check bash version before install

* Fix for non-zero exit status

Some environments have stringent security features that do not allow the
execution of functions inside `$PROMPT_COMMAND`, breaking every individual
theme.

We now allow the overriding of the prompt setter command to workaround
this.

Add 'bu' function to CD up faster

Only execute oh-my-bash on an interactive shell in .bashrc

First version of AWS plugin

Git plugin readme

Added README file for bashmarks plugin

Fixed issue #11

Fixed noise by rerouting the output of some external completion 

Fixed noise by rerouting the output of some external completion scripts to /dev/null fixing: ISSUE #2 from the master repository.

Fixes #4

Since spectrum.sh was copied from zsh, the typeset -A used is zsh specific and in bash4, it is declare -A. I've fixed it and I've added a check to error out with a readable message, in red color, to tell the user to upgrade bash.