Update Impoortmaps::Command#puts_table to be markdown compatible

[faqndo97]

Proposal to make the command table markdown compatible, so it's easier to copy the table and paste directly in any markdown (normally the PR description) and display it with the correct styles.

➜  dummy git:(main) ✗ bin/importmap outdated
+-------------------+---------+--------+
| Package           | Current | Latest |
+-------------------+---------+--------+
| @rails/request.js | 0.0.6   | 0.0.9  |
+-------------------+---------+--------+
  1 outdated package found
➜  dummy git:(main) ✗ bin/importmap outdated
| Package           | Current | Latest |
|-------------------|---------|--------|
| @rails/request.js | 0.0.6   | 0.0.9  |
  1 outdated package found

Also displaying the table here:

Old:
+-------------------+---------+--------+
| Package | Current | Latest |
+-------------------+---------+--------+
| @rails/request.js | 0.0.6 | 0.0.9 |
+-------------------+---------+--------+

New:

Package	Current	Latest
@rails/request.js	0.0.6	0.0.9

Another option instead of make this the default is to accept a flag like --md to print the table as markdown compatible.