New Feature: Import jrnl files

New feature added for importing jrnl files.

```
jekyll import jrnl
```

## Available overrides

- `--file`: Path to input file (default: ~/journal.txt)
- `--time_format`: the time format used by jrnl (default: %Y-%m-%d %H:%M)
- `--extension`: file extension of the output file (default: md)
- `--layout`: set layout of the output file (default: post)

## Usage

```
# will use ~/path/to/file/work.txt as the jrnl file
jekyll import jrnl --file ~/path/to/file/work.txt
```
```
# will output all files as txt files
jekyll import jrnl --extension txt
```
```
# will output all files with YAML data having layout: custom
jekyll import jrnl --layout custom
```