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

exit on error and note

Update README.md

grafana spell incorrectly.

Update README.md: Use --rm with docker run

Without --rm, subsequent runs will fail with an error such as:

    docker: Error response from daemon: Conflict. The container name "/grafana-backup-tool" is already in use by container "e8a6c8a7b717dd52402ea75c7189002bbb80836ae641df1e9865e0f1ec7835b7". You have to remove (or rename) that container to be able to reuse that name.                                                             
    See 'docker run --help'.

Preparing release v0.1.0, see CHANGELOG.md