[Bugfix] Remove hardcoding default branch as "master"

Default branch of repo could be "master", "main" or any names.
Stop hardcoding "master" as the default to clone and let "git"
decide it automatically.

Add branch support for plugins

Fixes #200

Fix hardcoded tmux.conf path in environment reload

Add support for 'source-file -q'

Use XDG-compatible plugin path when available

Reword the documentation file explaining how to change the path were
`tpm` installs its plugins to mention the new behaviour regarding
XDG_CONFIG_HOME.

If the `tmux.conf` file is found at a XDG_CONFIG_HOME, use this
directory to store the plugins instead of `$HONE/.tmux/`.

This is only effective if the user has not overridden the value of
`TMUX_PLUGIN_MANAGER_PATH` in their configuration file.

This reverts commit a403ca3b.

There are problems when running TPM with a `-b` flag, see #151.

Non-Github Plugin Install Fails due to Syntax Error in README.md

I thought it was a bit peculiar that the README.md suggested using `git@bitbucket.com/user/plugin`, but went ahead with that syntax. After seemingly endlessly scratching my head, while trying to figure out what the issue, I reverted to the traditional git URL syntax (SSH syntax) and it worked.

```
set -g @plugin 'git@bitbucket.com:user/plugin_name`
```

Verified by running `./.tmux/plugins/tpm/bin/install_plugins` manually with a fresh version of TMUX + TPM with both styles.

Starting from tmux 2.3, 'source-file' understands '-q' to suppress errors for
nonexistent files.

Default value for XDG_CONFIG_HOME

Update README.md

solution to the problem 'tpm returned 127'

Flexible TMux Configuration Load

It has been decided that this feature should been removed until it will be requested.

Update README.md installation instructions.

`run '~/.tmux/plugins/tpm/tpm'` should be run in the background, so the `-b` flag was added.

Add function `_get_user_tmux_conf` to helper script `plugin_functions`.
Function is searching for the users tmux configuration on multiple
places by a prioritized order.
The response is used within`_tmux_conf_contents` to read in the content
as normally.
Add new environment variable `TMUX_PLUGIN_MANAGER_CONFIG_LOCATION` which
is optional to be defined.
If so it has the highest priority to be loaded, despite if the file
exist or not.
XDG directory support has been added as well by the second priority
location at `$XDG_CONFIG_HOME/tmux/tmux.conf`.

correct spelling mistake

See issue #67 for more discussion

- Changed PNG Badges to SVG
- Added Styling to Code Blocks
- Added <kbd> tags for Key Bindings

Docs change to include auto plugin installation

Avoid any confusion with starting tmux with the `-f` option.

Fixes https://github.com/tmux-plugins/tpm/issues/89

If tmux was not yet configured to initialize the Tmux Plugin Manager,
the variable for the plugin home was not being set. This was resulting
in a `rm -rf /` which is pretty nuclear.

Fixes: https://github.com/tmux-plugins/tpm/issues/58