Update README.md

Some more minor mistakes that I missed..

Update README.md

1. Updated the list of the supported components of this tool.
2. Rewrote some text for better clarity.
3. Removed the specification of a Grafana version, as we are supporting every Grafana version there is (?).
4. Updated some URLs to official sites and fixed broken ones.

Added the ability to use multiple hard-coded settings

2) Use 'format' instead of f-string for compatibility with Python 2

Better explaination of this tool in README

2) Fixed import statements - don't use `from x import *`, but import only what is needed.
2) Reformatted the code.

try to fix version check chokes problem according to grafana's current version format. https://github.com/ysde/grafana-backup-tool/issues/37

Make GRAFANA_HEADERS environment optional

The current solution does not allow the GRAFANA_HEADERS to be optional. Pulling the if statement outside the dictionary comprehension to allow the variable to be optional.

add GRAFANA_HEADERS environment variable

fix(casting): cast int to str

search_api_limit can't be larger than 5000

The search_api_limit must not be larger than 5000 since April this year (grafana/grafana#16458). I figured this out today after upgrading grafana to 6.4.4 from 5.something.

Probably we should fix the search_dashboard() at some point to use pagination (for sites with more than 5000 dashboards, folders, datasource).

fix: uid doesn't exist on channel object