feat(toggle_checkbox): transform non-checkbox line into a checkbox

[anggerdeni]

toggle_checkbox function will transform a non-checkbox line into an (unchecked) checkbox by prepending - [ ] to the line. It will also respect whitespaces at the beginning of the line.

Sample cases:

before

some line
  some line with spacing

after

- [ ] some line
  - [ ] line with spacing

Closes #361

[epwalsh]

When testing I noticed that if the line starts with "-" (with no checkbox), this will add a 2nd "-". Can we handle that case too?

[anggerdeni]

Good point @epwalsh , that applies to all the other unordered list symbols as well. I've updated the code, the new behavior will simply change unordered list into checkbox

line1
 line2
  line3
- a
-  b
-   c
  - d
  -  e
  -   f
* g
*  h
*   i
  * j
  *  k
  *   l
+ m
+  n
+   o
  + p
  +  q
  +   r

will be transformed into

- [ ] line1
 - [ ] line2
  - [ ] line3
- [ ] a
- [ ]  b
- [ ]   c
  - [ ] d
  - [ ]  e
  - [ ]   f
- [ ] g
- [ ]  h
- [ ]   i
  - [ ] j
  - [ ]  k
  - [ ]   l
- [ ] m
- [ ]  n
- [ ]   o
  - [ ] p
  - [ ]  q
  - [ ]   r

[epwalsh]

LGTM! Thanks @anggerdeni